/* =========================================================
   Vật Lý Example24 — design tokens
   Indigo (khoa học, tin cậy) + cam (CTA), nền sáng dịu mắt.
   ========================================================= */
:root {
  --color-primary: #4338ca;
  --color-primary-hover: #3730a3;
  --color-primary-soft: #eef2ff;
  --color-on-primary: #ffffff;
  --color-accent: #ea580c;
  --color-accent-hover: #c2410c;
  --color-on-accent: #ffffff;
  --color-bg: #f7f8fc;
  --color-surface: #ffffff;
  --color-surface-alt: #eef1fa;
  --color-fg: #1e1b4b;
  --color-fg-soft: #334155;
  --color-muted: #5b6478;
  --color-border: #dfe3f0;
  --color-border-strong: #c7cde0;
  --color-success: #15803d;
  --color-success-soft: #dcfce7;
  --color-warning: #b45309;
  --color-warning-soft: #fef3c7;
  --color-danger: #dc2626;
  --color-danger-soft: #fee2e2;
  --color-info-soft: #e0f2fe;
  --color-ring: #6366f1;

  --level-thcs: #0e7490;
  --level-thcs-soft: #cffafe;
  --level-thpt: #4338ca;
  --level-thpt-soft: #e0e7ff;
  --level-on-thi: #c2410c;
  --level-on-thi-soft: #ffedd5;
  --level-dai-cuong: #7e22ce;
  --level-dai-cuong-soft: #f3e8ff;

  --font-heading: 'Be Vietnam Pro', 'Noto Sans', system-ui, sans-serif;
  --font-body: 'Noto Sans', system-ui, -apple-system, 'Segoe UI', sans-serif;

  --radius-sm: 8px;
  --radius: 12px;
  --radius-lg: 18px;
  --shadow-sm: 0 1px 2px rgb(30 27 75 / 0.06);
  --shadow: 0 1px 3px rgb(30 27 75 / 0.06), 0 6px 18px -6px rgb(30 27 75 / 0.12);
  --shadow-lg: 0 2px 6px rgb(30 27 75 / 0.06), 0 18px 40px -12px rgb(30 27 75 / 0.22);

  --space-1: 4px;
  --space-2: 8px;
  --space-3: 12px;
  --space-4: 16px;
  --space-5: 24px;
  --space-6: 32px;
  --space-7: 48px;
  --space-8: 64px;
  --space-9: 96px;

  --ease-out: cubic-bezier(0.22, 1, 0.36, 1);
  --container: 1180px;
}

/* ---------- Base ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font-body);
  font-size: 16px;
  line-height: 1.65;
  color: var(--color-fg-soft);
  background: var(--color-bg);
  text-rendering: optimizeLegibility;
}
img { max-width: 100%; height: auto; }
h1, h2, h3, h4 {
  font-family: var(--font-heading);
  color: var(--color-fg);
  line-height: 1.25;
  margin: 0 0 var(--space-3);
  text-wrap: balance;
}
h1 { font-size: clamp(1.75rem, 1.3rem + 2vw, 2.6rem); font-weight: 800; letter-spacing: -0.02em; }
h2 { font-size: clamp(1.35rem, 1.15rem + 0.9vw, 1.75rem); font-weight: 700; letter-spacing: -0.01em; }
h3, .h3 { font-size: 1.15rem; font-weight: 700; }
p { margin: 0 0 var(--space-4); }
a { color: var(--color-primary); text-underline-offset: 3px; }
a:hover { color: var(--color-primary-hover); }
code { font-family: ui-monospace, 'SFMono-Regular', Menlo, Consolas, monospace; font-size: 0.9em; background: var(--color-surface-alt); padding: 0.1em 0.35em; border-radius: 4px; }
:focus-visible { outline: 3px solid var(--color-ring); outline-offset: 2px; border-radius: 4px; }
main:focus { outline: none; }
[id] { scroll-margin-top: 88px; }

.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0, 0, 0, 0); white-space: nowrap; border: 0;
}
.skip-link {
  position: absolute; left: var(--space-4); top: -60px; z-index: 100;
  background: var(--color-fg); color: #fff; padding: var(--space-2) var(--space-4); border-radius: var(--radius-sm);
}
.skip-link:focus { top: var(--space-3); color: #fff; }

.icon {
  width: 20px; height: 20px; flex-shrink: 0;
  fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round;
}
.muted { color: var(--color-muted); }
.small { font-size: 0.875rem; }
.optional { font-weight: 400; color: var(--color-muted); font-size: 0.875rem; }
.lead { font-size: 1.125rem; color: var(--color-fg-soft); max-width: 62ch; }

.container { width: 100%; max-width: var(--container); margin: 0 auto; padding: 0 var(--space-4); }
.container.narrow { max-width: 820px; }
@media (min-width: 768px) { .container { padding: 0 var(--space-5); } }

/* ---------- Buttons ---------- */
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: var(--space-2);
  min-height: 44px; padding: 10px 18px;
  font: 600 0.95rem/1.2 var(--font-heading);
  border: 1.5px solid transparent; border-radius: var(--radius);
  cursor: pointer; text-decoration: none; white-space: nowrap;
  transition: background-color 180ms var(--ease-out), color 180ms var(--ease-out), border-color 180ms var(--ease-out), transform 120ms var(--ease-out), box-shadow 180ms var(--ease-out);
}
.btn:active { transform: translateY(1px); }
.btn .icon { width: 18px; height: 18px; }
.btn-sm { min-height: 40px; padding: 8px 12px; font-size: 0.875rem; }
.btn-lg { min-height: 52px; padding: 14px 24px; font-size: 1.05rem; }
.btn-block { width: 100%; }
.btn-primary { background: var(--color-primary); color: var(--color-on-primary); box-shadow: 0 4px 14px -4px rgb(67 56 202 / 0.55); }
.btn-primary:hover { background: var(--color-primary-hover); color: var(--color-on-primary); }
.btn-accent { background: var(--color-accent); color: var(--color-on-accent); box-shadow: 0 4px 14px -4px rgb(234 88 12 / 0.55); }
.btn-accent:hover { background: var(--color-accent-hover); color: var(--color-on-accent); }
.btn-secondary { background: var(--color-surface); color: var(--color-primary); border-color: var(--color-border-strong); }
.btn-secondary:hover { border-color: var(--color-primary); color: var(--color-primary-hover); background: var(--color-primary-soft); }
.btn-ghost { background: transparent; color: var(--color-fg-soft); }
.btn-ghost:hover { background: var(--color-surface-alt); color: var(--color-fg); }
.btn-danger { background: var(--color-danger); color: #fff; }
.btn-danger:hover { background: #b91c1c; color: #fff; }
.btn-danger-text { color: var(--color-danger); }
.btn-danger-text:hover { background: var(--color-danger-soft); color: #b91c1c; }

/* ---------- Header ---------- */
.site-header {
  position: sticky; top: 0; z-index: 50;
  background: rgb(255 255 255 / 0.92);
  backdrop-filter: saturate(1.4) blur(10px);
  border-bottom: 1px solid var(--color-border);
}
.header-inner { display: flex; align-items: center; gap: var(--space-4); min-height: 68px; }
.brand { display: inline-flex; align-items: center; gap: var(--space-2); text-decoration: none; color: var(--color-fg); }
.brand:hover { color: var(--color-fg); }
.brand-mark {
  display: grid; place-items: center; width: 38px; height: 38px; border-radius: 11px;
  background: linear-gradient(135deg, #4f46e5, #4338ca 60%, #312e81); color: #fff;
  box-shadow: 0 4px 12px -4px rgb(67 56 202 / 0.6);
}
.brand-mark .icon { width: 22px; height: 22px; }
.brand-text { white-space: nowrap; font: 600 1.1rem/1 var(--font-heading); letter-spacing: -0.01em; }
.brand-text strong { font-weight: 800; color: var(--color-primary); }

.nav-toggle {
  margin-left: auto; display: inline-grid; place-items: center; width: 44px; height: 44px;
  border: 1px solid var(--color-border); border-radius: var(--radius); background: var(--color-surface);
  color: var(--color-fg); cursor: pointer;
}
.site-nav {
  display: none; position: absolute; top: 100%; left: 0; right: 0;
  flex-direction: column; gap: var(--space-4); padding: var(--space-4);
  background: var(--color-surface); border-bottom: 1px solid var(--color-border); box-shadow: var(--shadow);
}
.site-nav.is-open { display: flex; }
.nav-links { list-style: none; margin: 0; padding: 0; display: flex; flex-direction: column; gap: 2px; }
.nav-links a {
  white-space: nowrap;
  display: flex; align-items: center; min-height: 44px; padding: 0 var(--space-3);
  border-radius: var(--radius-sm); text-decoration: none; color: var(--color-fg-soft); font-weight: 600;
}
.nav-links a:hover { background: var(--color-surface-alt); color: var(--color-fg); }
.nav-links a[aria-current='page'] { color: var(--color-primary); background: var(--color-primary-soft); }
.nav-search { position: relative; }
.nav-search .icon { position: absolute; left: 12px; top: 50%; transform: translateY(-50%); color: var(--color-muted); width: 18px; height: 18px; pointer-events: none; }
.nav-search input {
  width: 100%; min-height: 44px; padding: 0 var(--space-3) 0 40px;
  border: 1px solid var(--color-border); border-radius: var(--radius); background: var(--color-bg);
  font: inherit; color: var(--color-fg);
}
.nav-account { display: flex; flex-wrap: wrap; gap: var(--space-2); }
.nav-account form { margin: 0; }

@media (min-width: 1100px) {
  .nav-toggle { display: none; }
  .site-nav {
    display: flex; position: static; flex-direction: row; align-items: center; flex: 1;
    padding: 0; background: none; border: 0; box-shadow: none; gap: var(--space-4);
  }
  .nav-links { flex-direction: row; }
  .nav-links a { min-height: 40px; font-size: 0.95rem; }
  .nav-search { margin-left: auto; width: 200px; }
  .nav-account { flex-wrap: nowrap; }
}

/* ---------- Alerts ---------- */
.alert { margin: var(--space-4) 0; padding: var(--space-3) var(--space-4); border-radius: var(--radius); border: 1px solid; font-weight: 500; }
.alert-success { background: var(--color-success-soft); border-color: #86efac; color: #14532d; }
.alert-error { background: var(--color-danger-soft); border-color: #fca5a5; color: #7f1d1d; }
.alert-info { background: var(--color-info-soft); border-color: #7dd3fc; color: #0c4a6e; }

/* ---------- Hero ---------- */
.hero {
  position: relative; overflow: hidden;
  background:
    radial-gradient(900px 400px at 85% -10%, rgb(99 102 241 / 0.18), transparent 60%),
    radial-gradient(600px 300px at 0% 110%, rgb(234 88 12 / 0.10), transparent 60%),
    var(--color-surface);
  border-bottom: 1px solid var(--color-border);
}
.hero-inner { display: grid; gap: var(--space-7); padding-block: var(--space-8); align-items: center; }
.eyebrow {
  display: inline-block; margin-bottom: var(--space-4); padding: 6px 12px; border-radius: 999px;
  background: var(--color-primary-soft); color: var(--color-primary); font: 700 0.8rem/1 var(--font-heading); letter-spacing: 0.04em; text-transform: uppercase;
}
.hero h1 { font-size: clamp(2rem, 1.4rem + 3vw, 3.4rem); max-width: 16ch; }
.hero-actions { display: flex; flex-wrap: wrap; gap: var(--space-3); margin: var(--space-5) 0 var(--space-6); }
.hero-stats { display: flex; flex-wrap: wrap; gap: var(--space-6); margin: 0; }
.hero-stats div { display: flex; flex-direction: column-reverse; }
.hero-stats dt { color: var(--color-muted); font-size: 0.875rem; }
.hero-stats dd { margin: 0; font: 800 1.9rem/1.1 var(--font-heading); color: var(--color-fg); font-variant-numeric: tabular-nums; }

.hero-visual { position: relative; min-height: 320px; display: none; }
.hero-orbit { position: absolute; inset: 0; margin: auto; width: 82%; max-width: 380px; height: auto; }
.hero-orbit ellipse { fill: none; stroke: #a5b4fc; stroke-width: 1.5; }
.hero-orbit .nucleus { fill: var(--color-primary); }
.formula-card {
  position: absolute; z-index: 1; padding: var(--space-2) var(--space-4); border-radius: var(--radius);
  background: var(--color-surface); border: 1px solid var(--color-border); box-shadow: var(--shadow-lg); color: var(--color-fg);
}
.formula-card .katex-display { margin: 0; }
.formula-1 { top: 6%; left: 0; }
.formula-2 { bottom: 8%; right: 0; }
.formula-3 { top: 42%; right: 8%; }
@media (min-width: 900px) {
  .hero-inner { grid-template-columns: 1.15fr 1fr; padding-block: var(--space-9); }
  .hero-visual { display: block; }
}
@media (prefers-reduced-motion: no-preference) {
  .formula-card { animation: float 7s ease-in-out infinite; }
  .formula-2 { animation-delay: -2.3s; }
  .formula-3 { animation-delay: -4.6s; }
  .hero-orbit { animation: spin 60s linear infinite; }
}
@keyframes float { 50% { transform: translateY(-8px); } }
@keyframes spin { to { transform: rotate(360deg); } }

/* ---------- Sections & grids ---------- */
.section { padding-block: var(--space-8); }
.section-tight { padding-block: var(--space-6) var(--space-8); }
.section-alt { background: var(--color-surface-alt); }
.section-head { margin-bottom: var(--space-6); }
.section-head p { margin: 0; }
.section-title { font-size: 1.25rem; margin: var(--space-7) 0 var(--space-4); }
.grid { display: grid; gap: var(--space-4); }
@media (min-width: 640px) {
  .grid-2, .grid-3, .grid-4 { grid-template-columns: repeat(2, 1fr); }
}
@media (min-width: 1024px) {
  .grid { gap: var(--space-5); }
  .grid-3 { grid-template-columns: repeat(3, 1fr); }
  .grid-4 { grid-template-columns: repeat(4, 1fr); }
}

/* ---------- Cards ---------- */
.card {
  display: block; padding: var(--space-5); background: var(--color-surface);
  border: 1px solid var(--color-border); border-radius: var(--radius-lg); box-shadow: var(--shadow-sm);
  color: inherit; text-decoration: none;
}
a.card { transition: transform 200ms var(--ease-out), box-shadow 200ms var(--ease-out), border-color 200ms var(--ease-out); }
a.card:hover { color: inherit; border-color: var(--color-border-strong); box-shadow: var(--shadow-lg); transform: translateY(-2px); }
.card p { color: var(--color-muted); margin-bottom: 0; }
.card h3 { margin-top: var(--space-3); }

.level-card { display: flex; flex-direction: column; }
.level-card p { flex: 1; margin-bottom: var(--space-4); }
.level-icon {
  display: inline-grid; place-items: center; width: 48px; height: 48px; border-radius: 14px;
}
.level-icon .icon { width: 24px; height: 24px; }
.level-thcs { background: var(--level-thcs-soft); color: var(--level-thcs); }
.level-thpt { background: var(--level-thpt-soft); color: var(--level-thpt); }
.level-on-thi { background: var(--level-on-thi-soft); color: var(--level-on-thi); }
.level-dai-cuong { background: var(--level-dai-cuong-soft); color: var(--level-dai-cuong); }
.level-grades { margin-top: var(--space-4); font: 600 0.8rem/1 var(--font-heading); color: var(--color-muted); text-transform: uppercase; letter-spacing: 0.05em; }
.level-card h3 { margin-top: var(--space-1); font-size: 1.3rem; }
.level-meta { display: inline-flex; align-items: center; gap: var(--space-1); font: 600 0.9rem/1 var(--font-heading); color: var(--color-primary); }
.level-meta .icon { width: 16px; height: 16px; transition: transform 200ms var(--ease-out); }
a.card:hover .level-meta .icon { transform: translateX(3px); }

.feature-icon {
  display: inline-grid; place-items: center; width: 44px; height: 44px; border-radius: 12px;
  background: var(--color-primary-soft); color: var(--color-primary);
}
.feature h3 { margin-top: var(--space-3); }
.feature p { color: var(--color-muted); margin: 0; }
.sim-card .feature-icon { background: #ffedd5; color: var(--color-accent-hover); }
.sim-card h2, .sim-card h3 { margin-top: var(--space-3); }
.sim-card p { margin-bottom: var(--space-3); }

.badge {
  display: inline-flex; align-items: center; gap: 4px; padding: 4px 10px; border-radius: 999px;
  font: 600 0.78rem/1.3 var(--font-heading); background: var(--color-surface-alt); color: var(--color-fg-soft);
  text-decoration: none; white-space: nowrap;
}
.badge .icon { width: 14px; height: 14px; }
.badge-link:hover { background: var(--color-primary-soft); color: var(--color-primary); }
.badge-thcs { background: var(--level-thcs-soft); color: var(--level-thcs); }
.badge-thpt { background: var(--level-thpt-soft); color: var(--level-thpt); }
.badge-on-thi { background: var(--level-on-thi-soft); color: var(--level-on-thi); }
.badge-dai-cuong { background: var(--level-dai-cuong-soft); color: var(--level-dai-cuong); }
.badge-success { background: var(--color-success-soft); color: var(--color-success); }
.badge-warning { background: var(--color-warning-soft); color: var(--color-warning); }
.lesson-card h3 { margin-top: var(--space-3); }

.cta-band {
  display: flex; flex-direction: column; gap: var(--space-5); align-items: flex-start;
  padding: var(--space-6); border-radius: var(--radius-lg);
  background: linear-gradient(120deg, #312e81, #4338ca 55%, #6366f1); color: #e0e7ff;
}
.cta-band h2 { color: #fff; }
.cta-band p { margin: 0; }
@media (min-width: 768px) { .cta-band { flex-direction: row; align-items: center; justify-content: space-between; padding: var(--space-7); } }

/* ---------- Page head / breadcrumb ---------- */
.page-head { padding-block: var(--space-6) var(--space-5); background: var(--color-surface); border-bottom: 1px solid var(--color-border); }
.page-head h1 { margin-bottom: var(--space-2); }
.page-head .lead { margin-bottom: 0; }
.page-head-actions { margin-top: var(--space-4); }
.breadcrumb { display: flex; flex-wrap: wrap; gap: var(--space-2); margin-bottom: var(--space-3); font-size: 0.875rem; color: var(--color-muted); }
.breadcrumb a { color: var(--color-muted); text-decoration: none; }
.breadcrumb a:hover { color: var(--color-primary); text-decoration: underline; }
.page-title-row { display: flex; gap: var(--space-4); align-items: flex-start; }
.page-title-row .level-icon { margin-top: 4px; }
.title-sub { font-size: 0.55em; font-weight: 600; }

/* ---------- Chapter / lesson list ---------- */
.chapter + .chapter { margin-top: var(--space-7); }
.chapter-title { font-size: 1.3rem; }
.lesson-list { list-style: none; margin: var(--space-4) 0 0; padding: 0; display: grid; gap: var(--space-2); }
.lesson-row {
  display: flex; align-items: center; gap: var(--space-4); padding: var(--space-4);
  background: var(--color-surface); border: 1px solid var(--color-border); border-radius: var(--radius);
  text-decoration: none; color: inherit; transition: border-color 180ms var(--ease-out), box-shadow 180ms var(--ease-out);
}
.lesson-row:hover { color: inherit; border-color: var(--color-primary); box-shadow: var(--shadow); }
.lesson-status {
  display: grid; place-items: center; width: 28px; height: 28px; flex-shrink: 0;
  border-radius: 50%; border: 2px solid var(--color-border-strong); color: #fff;
}
.lesson-status .icon { width: 16px; height: 16px; stroke-width: 3; }
.is-done .lesson-status { background: var(--color-success); border-color: var(--color-success); }
.lesson-row-body { display: flex; flex-direction: column; flex: 1; min-width: 0; }
.lesson-row-title { font: 600 1rem/1.4 var(--font-heading); color: var(--color-fg); }
.lesson-row-summary { font-size: 0.875rem; color: var(--color-muted); }
.chevron { color: var(--color-muted); }
.chip { display: none; align-items: center; gap: 4px; padding: 4px 10px; border-radius: 999px; background: #ffedd5; color: var(--color-accent-hover); font: 600 0.75rem/1.3 var(--font-heading); }
.chip .icon { width: 14px; height: 14px; }
@media (min-width: 640px) { .chip { display: inline-flex; } }

.empty-state { text-align: center; padding: var(--space-8) var(--space-4); color: var(--color-muted); }
.empty-state > .icon { width: 48px; height: 48px; color: var(--color-border-strong); margin-bottom: var(--space-3); }

/* ---------- Lesson content ---------- */
.lesson-meta { display: flex; flex-wrap: wrap; gap: var(--space-2); margin-top: var(--space-4); }
.prose { padding-block: var(--space-6); font-size: 1.0625rem; max-width: 72ch; }
.prose h2 { margin-top: var(--space-6); font-size: 1.45rem; }
.prose h3 { margin-top: var(--space-5); }
.prose ul, .prose ol { padding-left: 1.4em; margin: 0 0 var(--space-4); }
.prose li { margin-bottom: var(--space-1); }
.prose li::marker { color: var(--color-primary); }
.prose blockquote {
  margin: var(--space-5) 0; padding: var(--space-3) var(--space-4);
  border-left: 4px solid var(--color-accent); background: #fff7ed; border-radius: 0 var(--radius) var(--radius) 0; color: #7c2d12;
}
.prose blockquote p:last-child { margin: 0; }
.prose table { width: 100%; border-collapse: collapse; margin: var(--space-4) 0; display: block; overflow-x: auto; }
.prose th, .prose td { padding: var(--space-2) var(--space-3); border: 1px solid var(--color-border); text-align: left; }
.prose th { background: var(--color-surface-alt); font-family: var(--font-heading); }
.prose img { border-radius: var(--radius); }
.prose details {
  margin: var(--space-4) 0; border: 1px solid var(--color-border); border-radius: var(--radius);
  background: var(--color-surface); overflow: hidden;
}
.prose summary {
  display: flex; align-items: center; gap: var(--space-2); min-height: 48px; padding: var(--space-2) var(--space-4);
  cursor: pointer; font: 600 0.95rem/1.4 var(--font-heading); color: var(--color-primary); list-style: none;
}
.prose summary::-webkit-details-marker { display: none; }
.prose summary::before { content: '▸'; transition: transform 150ms var(--ease-out); }
.prose details[open] summary::before { transform: rotate(90deg); }
.prose summary:hover { background: var(--color-primary-soft); }
.prose details > :not(summary) { margin-inline: var(--space-4); }
.prose details[open] { padding-bottom: var(--space-2); }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: var(--space-2) 0; }
.prose .katex-display { margin: var(--space-4) 0; padding: var(--space-3); background: var(--color-surface); border: 1px solid var(--color-border); border-radius: var(--radius); }
.katex { font-size: 1.1em; }

.lesson-section { margin-top: var(--space-6); padding-top: var(--space-6); border-top: 1px solid var(--color-border); }
.lesson-section > h2 { display: flex; align-items: center; gap: var(--space-2); }
.lesson-section > h2 .icon { color: var(--color-primary); width: 24px; height: 24px; }

/* ---------- Quiz ---------- */
.quiz-score { display: flex; flex-direction: column; gap: 2px; padding: var(--space-4); border-radius: var(--radius); margin-bottom: var(--space-5); border: 1px solid; }
.quiz-score strong { font: 700 1.2rem/1.3 var(--font-heading); }
.quiz-score.is-good { background: var(--color-success-soft); border-color: #86efac; color: #14532d; }
.quiz-score.is-ok { background: var(--color-warning-soft); border-color: #fcd34d; color: #78350f; }
.quiz-score.is-low { background: var(--color-danger-soft); border-color: #fca5a5; color: #7f1d1d; }
.question {
  margin: 0 0 var(--space-4); padding: var(--space-5); border: 1px solid var(--color-border);
  border-radius: var(--radius-lg); background: var(--color-surface); min-width: 0;
}
.question.is-correct { border-color: #86efac; }
.question.is-wrong { border-color: #fca5a5; }
.question legend { display: contents; }
.question-number { display: inline-block; margin-bottom: var(--space-2); font: 700 0.8rem/1 var(--font-heading); color: var(--color-primary); text-transform: uppercase; letter-spacing: 0.05em; }
.question-prompt { display: block; color: var(--color-fg); font-weight: 500; margin-bottom: var(--space-3); }
.question-prompt p { margin-bottom: var(--space-2); }
.options { display: grid; gap: var(--space-2); }
.option {
  position: relative; display: flex; align-items: center; gap: var(--space-3); min-height: 48px;
  padding: var(--space-2) var(--space-3); border: 1.5px solid var(--color-border); border-radius: var(--radius);
  cursor: pointer; transition: border-color 150ms var(--ease-out), background-color 150ms var(--ease-out);
}
.option:hover { border-color: var(--color-border-strong); background: var(--color-bg); }
.option input { position: absolute; opacity: 0; pointer-events: none; }
.option-letter {
  display: grid; place-items: center; width: 30px; height: 30px; flex-shrink: 0; border-radius: 8px;
  background: var(--color-surface-alt); color: var(--color-fg); font: 700 0.875rem/1 var(--font-heading);
}
.option:has(input:checked) { border-color: var(--color-primary); background: var(--color-primary-soft); }
.option:has(input:checked) .option-letter { background: var(--color-primary); color: #fff; }
.option:has(input:focus-visible) { outline: 3px solid var(--color-ring); outline-offset: 2px; }
.option:has(input:disabled) { cursor: default; }
.option.is-answer, .option.is-answer:has(input:checked) { border-color: var(--color-success); background: var(--color-success-soft); }
.option.is-answer .option-letter, .option.is-answer:has(input:checked) .option-letter { background: var(--color-success); color: #fff; }
.option.is-chosen-wrong, .option.is-chosen-wrong:has(input:checked) { border-color: var(--color-danger); background: var(--color-danger-soft); }
.option.is-chosen-wrong .option-letter, .option.is-chosen-wrong:has(input:checked) .option-letter { background: var(--color-danger); color: #fff; }
.option-tag { margin-left: auto; font: 600 0.75rem/1 var(--font-heading); white-space: nowrap; }
.question-verdict { margin: var(--space-3) 0 0; font-weight: 600; }
.is-correct .question-verdict { color: var(--color-success); }
.is-wrong .question-verdict { color: var(--color-danger); }
.explanation { margin-top: var(--space-2); padding: var(--space-3) var(--space-4); background: var(--color-bg); border-radius: var(--radius); }
.explanation p { display: inline; margin: 0; }
.quiz-actions { display: flex; gap: var(--space-3); }

.lesson-footer { margin-top: var(--space-7); padding-block: var(--space-6) var(--space-8); border-top: 1px solid var(--color-border); display: grid; gap: var(--space-5); }
.lesson-footer form { margin: 0; }
.pager { display: grid; gap: var(--space-3); }
@media (min-width: 640px) { .pager { grid-template-columns: 1fr 1fr; } }
.pager-link {
  display: flex; align-items: center; gap: var(--space-3); padding: var(--space-4);
  border: 1px solid var(--color-border); border-radius: var(--radius); background: var(--color-surface);
  text-decoration: none; color: var(--color-fg); font: 600 1rem/1.35 var(--font-heading);
}
.pager-link:hover { border-color: var(--color-primary); color: var(--color-primary); }
.pager-link small { display: block; font: 500 0.8rem/1.3 var(--font-body); color: var(--color-muted); }
.pager-next { justify-content: flex-end; text-align: right; }

/* ---------- Simulations ---------- */
.sim {
  display: grid; gap: var(--space-4); padding: var(--space-4); margin-top: var(--space-4);
  background: var(--color-surface); border: 1px solid var(--color-border); border-radius: var(--radius-lg); box-shadow: var(--shadow-sm);
}
@media (min-width: 960px) { .sim { grid-template-columns: 1.6fr 1fr; padding: var(--space-5); } }
.lesson .sim { grid-template-columns: 1fr; }
.sim-stage { border-radius: var(--radius); overflow: hidden; background: #f8faff; border: 1px solid var(--color-border); }
.sim-canvas { display: block; width: 100%; height: auto; aspect-ratio: 9 / 5; touch-action: manipulation; }
.sim-panel { display: flex; flex-direction: column; gap: var(--space-4); }
.sim-controls { display: grid; gap: var(--space-3); }
.slider { display: grid; grid-template-columns: 1fr auto; gap: 2px var(--space-3); font-weight: 600; font-size: 0.925rem; color: var(--color-fg); }
.slider > span { min-width: 0; }
.slider output { font-variant-numeric: tabular-nums; color: var(--color-primary); font-family: var(--font-heading); }
.slider input { grid-column: 1 / -1; width: 100%; height: 28px; accent-color: var(--color-primary); cursor: pointer; }
.check { display: flex; align-items: center; gap: var(--space-2); min-height: 44px; font-size: 0.925rem; cursor: pointer; }
.check input { width: 18px; height: 18px; accent-color: var(--color-primary); }
.preset-row { display: flex; flex-wrap: wrap; gap: var(--space-2); }
.chip-btn {
  min-height: 36px; padding: 6px 12px; border-radius: 999px; border: 1px solid var(--color-border-strong);
  background: var(--color-surface); font: 600 0.8rem/1 var(--font-heading); color: var(--color-fg-soft); cursor: pointer;
  transition: background-color 150ms var(--ease-out), border-color 150ms var(--ease-out);
}
.chip-btn:hover, .chip-btn[aria-pressed='true'] { border-color: var(--color-primary); background: var(--color-primary-soft); color: var(--color-primary); }
.segmented { display: inline-flex; padding: 4px; gap: 4px; border-radius: var(--radius); background: var(--color-surface-alt); }
.segmented label { flex: 1; cursor: pointer; }
.segmented input { position: absolute; opacity: 0; }
.segmented span { display: block; min-height: 38px; padding: 8px 14px; border-radius: var(--radius-sm); text-align: center; font: 600 0.875rem/1.4 var(--font-heading); color: var(--color-fg-soft); }
.segmented input:checked + span { background: var(--color-surface); color: var(--color-primary); box-shadow: var(--shadow-sm); }
.segmented input:focus-visible + span { outline: 3px solid var(--color-ring); outline-offset: 1px; }
.sim-readout { display: grid; grid-template-columns: 1fr 1fr; gap: var(--space-2); margin: 0; }
.sim-readout div { padding: var(--space-3); border-radius: var(--radius); background: var(--color-bg); border: 1px solid var(--color-border); }
.sim-readout dt { font-size: 0.8rem; color: var(--color-muted); line-height: 1.4; }
.sim-readout dd { margin: 2px 0 0; font: 700 1.15rem/1.3 var(--font-heading); color: var(--color-fg); font-variant-numeric: tabular-nums; }
.sim-buttons { display: flex; gap: var(--space-2); }
.related { margin-top: var(--space-6); }

/* ---------- Search ---------- */
.search-form { display: flex; gap: var(--space-2); margin-top: var(--space-4); }
.search-form input {
  flex: 1; min-width: 0; min-height: 48px; padding: 0 var(--space-4); font: inherit;
  border: 1.5px solid var(--color-border-strong); border-radius: var(--radius); background: var(--color-surface);
}

/* ---------- Progress ---------- */
.stat { display: flex; flex-direction: column; gap: var(--space-1); }
.stat-label { color: var(--color-muted); font-size: 0.875rem; }
.stat-value { font: 800 2rem/1.1 var(--font-heading); color: var(--color-fg); font-variant-numeric: tabular-nums; }
.progress-head { display: flex; align-items: center; gap: var(--space-3); margin-bottom: var(--space-3); }
.progress-head h3 { margin: 0; }
.progress-pct { margin-left: auto; font: 800 1.25rem/1 var(--font-heading); color: var(--color-fg); }
.progress-bar { height: 10px; border-radius: 999px; background: var(--color-surface-alt); overflow: hidden; }
.progress-bar span { display: block; height: 100%; border-radius: inherit; background: linear-gradient(90deg, #6366f1, var(--color-primary)); transition: width 600ms var(--ease-out); }
.progress-lists { margin-top: var(--space-2); }
.simple-list { list-style: none; margin: 0; padding: 0; display: grid; gap: var(--space-2); }
.simple-list li { display: flex; align-items: center; gap: var(--space-2); flex-wrap: wrap; }
.simple-list .muted { margin-left: auto; }
.icon-success { color: var(--color-success); }
.score { font-weight: 700; font-variant-numeric: tabular-nums; }
.score.is-good { color: var(--color-success); }
.score.is-ok { color: var(--color-warning); }
.score.is-low { color: var(--color-danger); }

/* ---------- Tables ---------- */
.table-wrap { overflow-x: auto; border: 1px solid var(--color-border); border-radius: var(--radius-lg); background: var(--color-surface); }
.table { width: 100%; border-collapse: collapse; font-size: 0.925rem; }
.table th, .table td { padding: var(--space-3) var(--space-4); text-align: left; border-bottom: 1px solid var(--color-border); vertical-align: middle; }
.table th { font: 600 0.8rem/1.3 var(--font-heading); color: var(--color-muted); text-transform: uppercase; letter-spacing: 0.04em; background: var(--color-bg); }
.table tbody tr:last-child td { border-bottom: 0; }
.table tbody tr:hover { background: var(--color-bg); }
.row-actions { display: flex; gap: var(--space-1); justify-content: flex-end; flex-wrap: wrap; }
.row-actions form { margin: 0; }

/* ---------- Forms ---------- */
.field { display: flex; flex-direction: column; gap: 6px; margin: 0 0 var(--space-4); border: 0; padding: 0; min-width: 0; }
.field label, .field legend, .label { font: 600 0.925rem/1.4 var(--font-heading); color: var(--color-fg); }
.field legend { margin-bottom: 6px; }
.field input[type='text'], .field input[type='email'], .field input[type='password'], .field input[type='number'], .field select, .field textarea, .inline-form select {
  width: 100%; min-height: 46px; padding: 10px 14px; font: inherit; font-size: 1rem; color: var(--color-fg);
  background: var(--color-surface); border: 1.5px solid var(--color-border-strong); border-radius: var(--radius);
  transition: border-color 150ms var(--ease-out), box-shadow 150ms var(--ease-out);
}
.field textarea { line-height: 1.55; resize: vertical; }
.field input:focus, .field select:focus, .field textarea:focus { border-color: var(--color-primary); box-shadow: 0 0 0 3px rgb(99 102 241 / 0.2); outline: none; }
.has-error input, .has-error select, .has-error textarea { border-color: var(--color-danger); }
.password-field { position: relative; }
.password-field input { padding-right: 52px !important; }
.password-toggle {
  position: absolute; top: 50%; right: 4px; transform: translateY(-50%);
  display: grid; place-items: center; width: 44px; height: 40px;
  border: 0; border-radius: var(--radius-sm); background: transparent; color: var(--color-muted); cursor: pointer;
  transition: color 150ms var(--ease-out), background-color 150ms var(--ease-out);
}
.password-toggle:hover { color: var(--color-primary); background: var(--color-primary-soft); }
.password-toggle[aria-pressed='true'] { color: var(--color-primary); }
.field-error { margin: 0; color: var(--color-danger); font-size: 0.875rem; font-weight: 500; }
.field-hint { margin: 0; color: var(--color-muted); font-size: 0.85rem; }
.form-grid { display: grid; gap: 0 var(--space-4); }
@media (min-width: 768px) { .form-grid { grid-template-columns: 1fr 1fr; } .field-wide { grid-column: 1 / -1; } }
.form-actions { display: flex; flex-wrap: wrap; gap: var(--space-2); margin-top: var(--space-2); }
.sticky-actions { position: sticky; bottom: 0; z-index: 5; padding: var(--space-3) 0; background: linear-gradient(transparent, var(--color-bg) 30%); }
.form-card { padding: var(--space-5); }
.form-card h2 { margin-bottom: var(--space-4); }
.inline-form { display: flex; gap: var(--space-2); align-items: center; margin: 0; }
.inline-form select { min-height: 40px; padding: 6px 10px; width: auto; }

.auth { display: grid; place-items: center; padding-inline: var(--space-4); }
.auth-card { width: 100%; max-width: 440px; padding: var(--space-6); box-shadow: var(--shadow-lg); }
.auth-card h1 { font-size: 1.75rem; }
.auth-card > p.muted { margin-bottom: var(--space-5); }
.auth-switch { margin: var(--space-5) 0 0; text-align: center; color: var(--color-muted); }

/* ---------- Admin ---------- */
.admin-head h1 { margin-bottom: var(--space-4); }
.admin-nav { display: flex; gap: var(--space-1); overflow-x: auto; }
.admin-nav a {
  display: inline-flex; align-items: center; gap: var(--space-2); min-height: 42px; padding: 0 var(--space-4);
  border-radius: var(--radius); text-decoration: none; color: var(--color-fg-soft); font: 600 0.925rem/1 var(--font-heading); white-space: nowrap;
}
.admin-nav a:hover { background: var(--color-surface-alt); }
.admin-nav a[aria-current='page'] { background: var(--color-primary-soft); color: var(--color-primary); }
.admin-nav .icon { width: 18px; height: 18px; }
.toolbar { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: var(--space-3); margin-bottom: var(--space-4); }
.toolbar .section-title, .toolbar h1 { margin: 0; }
.stat-row + .toolbar { margin-top: var(--space-6); }
.admin-split { display: grid; gap: var(--space-5); align-items: start; }
@media (min-width: 1024px) { .admin-split { grid-template-columns: 1fr 360px; } }

.editor-label-row { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: var(--space-2); }
.help summary { cursor: pointer; color: var(--color-primary); font-size: 0.875rem; font-weight: 600; min-height: 32px; display: flex; align-items: center; }
.help-body { padding: var(--space-3); margin-top: var(--space-2); border-radius: var(--radius); background: var(--color-surface); border: 1px solid var(--color-border); font-size: 0.875rem; }
.help-body p { margin: 0 0 var(--space-2); }
.editor { display: grid; gap: var(--space-3); }
@media (min-width: 1024px) { .editor { grid-template-columns: 1fr 1fr; } }
.editor textarea { font-family: ui-monospace, 'SFMono-Regular', Menlo, Consolas, monospace; font-size: 0.9rem; min-height: 420px; }
.editor-preview {
  padding: var(--space-4); min-height: 200px; max-height: 640px; overflow: auto;
  border: 1.5px dashed var(--color-border-strong); border-radius: var(--radius); background: var(--color-surface);
}
.editor-preview:empty::before { content: 'Bản xem trước sẽ hiện ở đây…'; color: var(--color-muted); }
.editor-preview.prose { padding-block: var(--space-4); font-size: 1rem; }

.admin-questions { margin-top: var(--space-7); }
.admin-question-list { list-style: none; padding: 0; margin: 0; display: grid; gap: var(--space-3); }
.admin-question-list .card { padding: var(--space-4); }
.admin-options { list-style: none; padding: 0; margin: var(--space-2) 0; display: grid; gap: 2px; font-size: 0.925rem; }
.admin-options li { display: flex; align-items: center; gap: var(--space-1); }
.admin-options .is-answer { color: var(--color-success); font-weight: 600; }
.admin-options .icon { width: 16px; height: 16px; }
.option-input { display: flex; align-items: center; gap: var(--space-2); margin-bottom: var(--space-2); }
.option-input input[type='radio'] { width: 22px; height: 22px; accent-color: var(--color-success); flex-shrink: 0; cursor: pointer; }
.option-input .option-letter { font-size: 0.875rem; }
.option-input input[type='text'] {
  flex: 1; min-width: 0; min-height: 44px; padding: 8px 12px; font: inherit;
  border: 1.5px solid var(--color-border-strong); border-radius: var(--radius);
}
.danger-zone { margin-top: var(--space-6); padding-top: var(--space-5); border-top: 1px dashed var(--color-border-strong); }

/* ---------- Footer ---------- */
.site-footer { background: var(--color-surface); border-top: 1px solid var(--color-border); padding-top: var(--space-7); }
.footer-inner { display: grid; gap: var(--space-6); }
@media (min-width: 768px) { .footer-inner { grid-template-columns: 2fr 1fr 1fr; } }
.footer-inner p { margin-top: var(--space-3); max-width: 40ch; }
.footer-title { font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.06em; color: var(--color-muted); }
.site-footer ul { list-style: none; margin: 0; padding: 0; display: grid; gap: var(--space-1); }
.site-footer ul a { color: var(--color-fg-soft); text-decoration: none; display: inline-block; padding: 4px 0; }
.site-footer ul a:hover { color: var(--color-primary); }
.footer-bottom { padding-block: var(--space-5); margin-top: var(--space-6); border-top: 1px solid var(--color-border); color: var(--color-muted); }

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: 0.01ms !important; animation-duration: 0.01ms !important; scroll-behavior: auto !important; }
}

/* ---------- Mô phỏng: trang danh mục ---------- */
.section-more { margin-top: var(--space-5); display: flex; justify-content: center; }
.chip-nav { display: flex; flex-wrap: wrap; gap: var(--space-2); margin-top: var(--space-4); }
.chip-link {
  display: inline-flex; align-items: center; gap: var(--space-2); min-height: 40px; padding: 6px 14px;
  border-radius: 999px; border: 1px solid var(--color-border-strong); background: var(--color-surface);
  font: 600 0.875rem/1.2 var(--font-heading); color: var(--color-fg-soft); text-decoration: none;
}
.chip-link:hover { border-color: var(--color-primary); color: var(--color-primary); background: var(--color-primary-soft); }
.chip-link span { display: inline-grid; place-items: center; min-width: 22px; height: 22px; border-radius: 999px; background: var(--color-surface-alt); font-size: 0.75rem; }
.sim-group + .sim-group { margin-top: var(--space-7); }
.sim-group-title { font-size: 1.35rem; margin-bottom: var(--space-4); }
.sim-card-full { display: flex; flex-direction: column; }
.sim-card-full > p { margin-bottom: var(--space-3); }
.param-list { list-style: none; display: flex; flex-wrap: wrap; gap: 6px; margin: 0 0 var(--space-3); padding: 0; }
.param-list li { padding: 3px 10px; border-radius: 999px; background: var(--color-primary-soft); color: var(--color-primary); font: 600 0.75rem/1.5 var(--font-heading); }
.card p.sim-apps { font-size: 0.875rem; color: var(--color-fg-soft); flex: 1; }
.sim-apps strong { color: var(--color-fg); }
.sim-open { align-self: flex-start; margin-top: var(--space-2); }
.sim-title-icon { width: 52px; height: 52px; flex-shrink: 0; margin-top: 6px; background: #ffedd5; color: var(--color-accent-hover); }
.sim-title-icon .icon { width: 26px; height: 26px; }
.sim-info { margin-top: var(--space-5); }
.sim-info .card p { margin: 0; }
.sim-info .simple-list .icon { width: 18px; height: 18px; color: var(--color-primary); }

/* ---------- Mô phỏng: chi tiết ---------- */
.sim-hint { margin: 0; padding: var(--space-2) var(--space-3); font-size: 0.85rem; color: var(--color-muted); border-top: 1px solid var(--color-border); background: var(--color-surface); }
.sim-note { margin: 0; font-size: 0.85rem; color: var(--color-muted); }
.sim-readout dd.readout-text { font: 600 0.9rem/1.4 var(--font-body); }
.slider input:disabled { opacity: 0.4; cursor: not-allowed; }
.segmented { display: flex; }