/* ==========================================================================
   Sổ tay SGK — giao diện dùng chung
   Mọi màu sắc là biến trên :root; chế độ tối định nghĩa lại bên dưới.
   ========================================================================== */

:root {
  --bg: #f6f7fb;
  --surface: #ffffff;
  --surface-2: #f0f2f7;
  --text: #1c2230;
  --text-muted: #5b6475;
  --border: #dde1ea;
  --shadow: 0 1px 2px rgba(20, 30, 50, .06), 0 4px 16px rgba(20, 30, 50, .06);

  --brand: #2f5bea;
  --brand-soft: #e7edff;

  /* Màu môn (trang có thể ghi đè --accent) */
  --ly: #2563eb;      --ly-soft: #e6efff;
  --hoa: #d9480f;     --hoa-soft: #fff0e6;
  --sinh: #2b8a3e;    --sinh-soft: #e7f6ea;
  --en: #7c3aed;      --en-soft: #f1ebff;
  --t1: #0f766e;      --t1-soft: #e2f5f2;
  --t2: #be185d;      --t2-soft: #fdebf3;
  --ls: #b45309;      --ls-soft: #fdf1e0;
  --dl: #047857;      --dl-soft: #e3f5ee;
  --cd: #1d4ed8;      --cd-soft: #e8eeff;
  --vm: #7e22ce;      --vm-soft: #f5ebff;
  --hsg: #b45309;     --hsg-soft: #fef3c7;
  --outline: #0369a1; --outline-bg: #eef8ff;
  --accent: var(--brand);
  --accent-soft: var(--brand-soft);

  /* Màu hộp kiến thức */
  --def: #2563eb;     --def-bg: #eef4ff;
  --law: #7c3aed;     --law-bg: #f4efff;
  --note: #b7791f;    --note-bg: #fff8e6;
  --tip: #2f855a;     --tip-bg: #ebf8f0;
  --warn: #c53030;    --warn-bg: #fff0f0;
  --ex: #4a5568;      --ex-bg: #f3f4f7;
  --formula: #c2410c; --formula-bg: #fffaf3;

  --on-accent: #ffffff;
  --radius: 12px;
  --font: "Be Vietnam Pro", "Segoe UI", system-ui, -apple-system, Roboto, Arial, sans-serif;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #11151d; --surface: #1a1f2a; --surface-2: #222836;
    --text: #e6e9f0; --text-muted: #9aa3b5; --border: #2e3545;
    --shadow: 0 1px 2px rgba(0,0,0,.3), 0 4px 16px rgba(0,0,0,.25);
    --brand: #7c9cff; --brand-soft: #1f2a4a;
    --ly: #6ea0ff; --ly-soft: #1b2a47;
    --hoa: #ff8a50; --hoa-soft: #3a2418;
    --sinh: #5fd07a; --sinh-soft: #183322;
    --en: #b595ff; --en-soft: #261d3d;
    --t1: #3fd0bd; --t1-soft: #14302d;
    --t2: #f47bb5; --t2-soft: #3a1a2a;
    --ls: #f0a83a; --ls-soft: #332512;
    --dl: #45c99a; --dl-soft: #122d25;
    --cd: #7c9cff; --cd-soft: #1c2745;
    --vm: #c38bff; --vm-soft: #2a1c3d;
    --hsg: #fbbf24; --hsg-soft: #33270f;
    --outline: #5cc0f2; --outline-bg: #12263a;
    --def: #7aa7ff; --def-bg: #172238;
    --law: #b595ff; --law-bg: #231c3a;
    --note: #f0b94a; --note-bg: #2e2614;
    --tip: #6ad49a; --tip-bg: #15291f;
    --warn: #ff7b7b; --warn-bg: #331919;
    --ex: #b8c0cf; --ex-bg: #222836;
    --formula: #ff9a5c; --formula-bg: #2a1f17;
    --on-accent: #11151d;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #11151d; --surface: #1a1f2a; --surface-2: #222836;
  --text: #e6e9f0; --text-muted: #9aa3b5; --border: #2e3545;
  --shadow: 0 1px 2px rgba(0,0,0,.3), 0 4px 16px rgba(0,0,0,.25);
  --brand: #7c9cff; --brand-soft: #1f2a4a;
  --ly: #6ea0ff; --ly-soft: #1b2a47;
  --hoa: #ff8a50; --hoa-soft: #3a2418;
  --sinh: #5fd07a; --sinh-soft: #183322;
  --en: #b595ff; --en-soft: #261d3d;
  --t1: #3fd0bd; --t1-soft: #14302d;
  --t2: #f47bb5; --t2-soft: #3a1a2a;
  --ls: #f0a83a; --ls-soft: #332512;
  --dl: #45c99a; --dl-soft: #122d25;
  --cd: #7c9cff; --cd-soft: #1c2745;
  --vm: #c38bff; --vm-soft: #2a1c3d;
  --hsg: #fbbf24; --hsg-soft: #33270f;
  --outline: #5cc0f2; --outline-bg: #12263a;
  --def: #7aa7ff; --def-bg: #172238;
  --law: #b595ff; --law-bg: #231c3a;
  --note: #f0b94a; --note-bg: #2e2614;
  --tip: #6ad49a; --tip-bg: #15291f;
  --warn: #ff7b7b; --warn-bg: #331919;
  --ex: #b8c0cf; --ex-bg: #222836;
  --formula: #ff9a5c; --formula-bg: #2a1f17;
  --on-accent: #11151d;
  color-scheme: dark;
}

/* ---------- Nền tảng ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 76px; }
body {
  margin: 0; background: var(--bg); color: var(--text);
  font-family: var(--font); font-size: 16.5px; line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); }
img, svg { max-width: 100%; }
h1, h2, h3, h4 { line-height: 1.3; margin: 0; }
p { margin: .5em 0; }
ul, ol { margin: .4em 0; padding-left: 1.3em; }
li { margin: .2em 0; }
b, strong { font-weight: 650; }
.muted { color: var(--text-muted); }

/* ---------- Thanh trên cùng ---------- */
.topbar {
  position: sticky; top: 0; z-index: 50;
  background: color-mix(in srgb, var(--surface) 88%, transparent);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border);
}
.topbar-inner {
  max-width: 1280px; margin: 0 auto; padding: 10px 16px;
  display: flex; align-items: center; gap: 12px;
}
.logo { display: flex; align-items: center; gap: 8px; font-weight: 750; color: var(--text); text-decoration: none; font-size: 17px; }
.logo-mark { width: 30px; height: 30px; border-radius: 8px; background: var(--brand); color: #fff; display: grid; place-items: center; font-size: 16px; }
.crumbs { display: flex; gap: 6px; align-items: center; color: var(--text-muted); font-size: 14px; flex-wrap: wrap; min-width: 0; }
.crumbs a { color: var(--text-muted); text-decoration: none; }
.crumbs a:hover { color: var(--text); }
.topbar .spacer { flex: 1; }
.icon-btn {
  border: 1px solid var(--border); background: var(--surface); color: var(--text);
  border-radius: 8px; height: 34px; min-width: 34px; padding: 0 10px; cursor: pointer; font: inherit; font-size: 14px;
}
.icon-btn:hover { background: var(--surface-2); }
.topbar .icon-btn { flex-shrink: 0; }
@media (max-width: 520px) {
  .topbar .crumbs { display: none; }
}

/* ---------- Trang chủ & trang lớp ---------- */
.hero { max-width: 1080px; margin: 0 auto; padding: 48px 16px 16px; }
.hero h1 { font-size: clamp(28px, 4vw, 40px); font-weight: 800; letter-spacing: -.01em; }
.hero p { color: var(--text-muted); font-size: 17px; max-width: 640px; }
.grid-wrap { max-width: 1080px; margin: 0 auto; padding: 8px 16px 64px; }
.group-title { font-size: 13px; text-transform: uppercase; letter-spacing: .08em; color: var(--text-muted); margin: 28px 0 12px; font-weight: 700; }
.card-grid { display: grid; gap: 14px; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); }
.card {
  position: relative; display: flex; flex-direction: column; gap: 6px;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 18px; text-decoration: none; color: var(--text); box-shadow: var(--shadow);
  transition: transform .15s ease, border-color .15s ease;
}
a.card:hover { transform: translateY(-2px); border-color: var(--card-color, var(--brand)); }
.card-icon {
  width: 46px; height: 46px; border-radius: 12px; display: grid; place-items: center; font-size: 24px;
  background: color-mix(in srgb, var(--card-color, var(--brand)) 16%, transparent); color: var(--card-color, var(--brand)); font-weight: 800;
}
.card-title { font-weight: 700; font-size: 18px; margin-top: 6px; }
.card-desc { color: var(--text-muted); font-size: 14px; }
.card.disabled { opacity: .55; box-shadow: none; }
.badge { display: inline-block; font-size: 12px; padding: 2px 8px; border-radius: 99px; background: var(--surface-2); color: var(--text-muted); font-weight: 600; width: fit-content; }
.badge.ready { background: var(--tip-bg); color: var(--tip); }

/* ---------- Trang môn học ---------- */
.subject-head { max-width: 1280px; margin: 0 auto; padding: 28px 16px 8px; }
.subject-head h1 { font-size: clamp(24px, 3.4vw, 34px); font-weight: 800; }
.subject-head p { color: var(--text-muted); margin: 4px 0 0; }

.tabs {
  max-width: 1280px; margin: 0 auto; padding: 12px 16px 0;
  display: flex; gap: 8px; overflow-x: auto; border-bottom: 1px solid var(--border);
}
.tab {
  --c: var(--brand); --cs: var(--brand-soft);
  border: 0; background: none; font: inherit; font-weight: 650; cursor: pointer; color: var(--text-muted);
  padding: 10px 16px; border-radius: 10px 10px 0 0; border-bottom: 3px solid transparent; white-space: nowrap;
  display: flex; align-items: center; gap: 8px;
}
.tab:hover { color: var(--text); background: var(--surface-2); }
.tab[aria-selected="true"] { color: var(--c); border-bottom-color: var(--c); background: var(--cs); }
.tab-dot { width: 10px; height: 10px; border-radius: 50%; background: var(--c); }
.tab.ly   { --c: var(--ly);   --cs: var(--ly-soft); }
.tab.hoa  { --c: var(--hoa);  --cs: var(--hoa-soft); }
.tab.sinh { --c: var(--sinh); --cs: var(--sinh-soft); }
.tab.t1   { --c: var(--t1);   --cs: var(--t1-soft); }
.tab.t2   { --c: var(--t2);   --cs: var(--t2-soft); }
.tab.ls   { --c: var(--ls);   --cs: var(--ls-soft); }
.tab.dl   { --c: var(--dl);   --cs: var(--dl-soft); }
.tab.cd   { --c: var(--cd);   --cs: var(--cd-soft); }
.tab.vm   { --c: var(--vm);   --cs: var(--vm-soft); }
.tab.hsg  { --c: var(--hsg);  --cs: var(--hsg-soft); }
.tab[disabled] { opacity: .45; cursor: default; }

.panel.ly   { --accent: var(--ly);   --accent-soft: var(--ly-soft); }
.panel.hoa  { --accent: var(--hoa);  --accent-soft: var(--hoa-soft); }
.panel.sinh { --accent: var(--sinh); --accent-soft: var(--sinh-soft); }
.panel.en   { --accent: var(--en);   --accent-soft: var(--en-soft); }
.panel.t1   { --accent: var(--t1);   --accent-soft: var(--t1-soft); }
.panel.t2   { --accent: var(--t2);   --accent-soft: var(--t2-soft); }
.panel.ls   { --accent: var(--ls);   --accent-soft: var(--ls-soft); }
.panel.dl   { --accent: var(--dl);   --accent-soft: var(--dl-soft); }
.panel.cd   { --accent: var(--cd);   --accent-soft: var(--cd-soft); }
.panel.vm   { --accent: var(--vm);   --accent-soft: var(--vm-soft); }
.panel.hsg  { --accent: var(--hsg);  --accent-soft: var(--hsg-soft); }
.panel[hidden] { display: none; }

.layout {
  max-width: 1280px; margin: 0 auto; padding: 20px 16px 80px;
  display: grid; grid-template-columns: 270px minmax(0, 1fr); gap: 28px; align-items: start;
}
.toc {
  position: sticky; top: 72px; max-height: calc(100vh - 88px); overflow: auto;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 14px 12px; font-size: 14px;
}
.toc-title { font-size: 12px; text-transform: uppercase; letter-spacing: .08em; color: var(--text-muted); font-weight: 700; margin: 0 6px 8px; }
.toc input {
  width: 100%; padding: 7px 10px; border-radius: 8px; border: 1px solid var(--border);
  background: var(--surface-2); color: var(--text); font: inherit; font-size: 14px; margin-bottom: 8px;
}
.toc ol { list-style: none; padding: 0; margin: 0; }
.toc .toc-chapter { font-weight: 700; color: var(--accent); margin: 10px 6px 2px; font-size: 12.5px; text-transform: uppercase; letter-spacing: .03em; }
.toc .katex { text-transform: none; font-size: 1em; }
.toc a { display: block; padding: 4px 8px; border-radius: 6px; color: var(--text); text-decoration: none; line-height: 1.4; }
.toc a:hover { background: var(--surface-2); }
.toc a.active { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.toc-toggle { display: none; }

/* ---------- Chương & bài ---------- */
.chapter { margin-bottom: 40px; }
.chapter-title {
  font-size: 22px; font-weight: 800; padding: 14px 18px; border-radius: var(--radius);
  background: var(--accent); color: var(--on-accent); margin-bottom: 18px;
}
.chapter-title span { display: block; font-size: 12.5px; font-weight: 700; text-transform: uppercase; letter-spacing: .1em; opacity: .85; }

.lesson {
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 22px 24px; margin-bottom: 20px; box-shadow: var(--shadow);
}
.lesson-title { font-size: 21px; font-weight: 750; display: flex; gap: 10px; align-items: baseline; flex-wrap: wrap; margin-bottom: 6px; }
.lesson-no {
  font-size: 13px; font-weight: 800; color: var(--accent); background: var(--accent-soft);
  padding: 3px 10px; border-radius: 99px; white-space: nowrap;
}
.lesson-goal { color: var(--text-muted); font-size: 15px; border-left: 3px solid var(--border); padding-left: 10px; margin: 6px 0 14px; }
.lesson h4 { font-size: 17px; margin: 22px 0 8px; color: var(--accent); }
.lesson h4:first-of-type { margin-top: 14px; }
.lesson h5 { font-size: 15.5px; margin: 14px 0 4px; }
.hl { background: color-mix(in srgb, var(--note) 22%, transparent); padding: 0 3px; border-radius: 3px; }
.kw { color: var(--accent); font-weight: 650; }

/* ---------- Hộp kiến thức ---------- */
.box {
  --c: var(--ex); --b: var(--ex-bg);
  border: 1px solid color-mix(in srgb, var(--c) 30%, transparent);
  border-left: 5px solid var(--c); background: var(--b);
  border-radius: 10px; padding: 12px 16px; margin: 14px 0;
}
.box::before {
  content: var(--label); display: inline-block; font-size: 11.5px; font-weight: 800; letter-spacing: .08em;
  color: var(--c); text-transform: uppercase; margin-bottom: 2px;
}
.box-title { font-weight: 700; margin: 0 0 4px; }
.box > :last-child { margin-bottom: 0; }
.box.def     { --c: var(--def);  --b: var(--def-bg);  --label: "📘 Định nghĩa"; }
.box.law     { --c: var(--law);  --b: var(--law-bg);  --label: "⚖️ Định luật · Quy tắc"; }
.box.note    { --c: var(--note); --b: var(--note-bg); --label: "📝 Ghi chú"; }
.box.tip     { --c: var(--tip);  --b: var(--tip-bg);  --label: "💡 Mẹo nhớ"; }
.box.warn    { --c: var(--warn); --b: var(--warn-bg); --label: "⚠️ Dễ nhầm"; }
.box.example { --c: var(--ex);   --b: var(--ex-bg);   --label: "✏️ Ví dụ"; }
.box.method  { --c: var(--t1);   --b: var(--t1-soft); --label: "🧭 Cách giải"; }
.box.fact    { --c: var(--def);  --b: var(--surface-2); --label: "🔎 Tính chất · Đặc điểm"; }
.box[data-label]::before { content: attr(data-label); }

/* Công thức đóng khung */
.formula {
  border: 2px solid var(--formula); background: var(--formula-bg);
  border-radius: 12px; padding: 12px 18px 14px; margin: 16px 0; position: relative;
}
.formula-name {
  display: inline-block; font-size: 12.5px; font-weight: 800; letter-spacing: .04em;
  color: var(--on-accent); background: var(--formula); padding: 2px 10px; border-radius: 6px; margin-bottom: 4px;
}
.formula-body { font-size: calc(1.22em * var(--fit, 1)); text-align: center; overflow-x: auto; overflow-y: hidden; padding: 4px 0; }
.formula-body .katex-display { margin: .4em 0; }
.formula-legend {
  list-style: none; padding: 8px 0 0; margin: 6px 0 0; border-top: 1px dashed color-mix(in srgb, var(--formula) 45%, transparent);
  display: grid; grid-template-columns: repeat(auto-fit, minmax(min(210px, 100%), 1fr)); gap: 2px 18px; font-size: 14.5px;
}
.formula-legend li { margin: 0; }
.formula-note { font-size: 14px; color: var(--text-muted); margin-top: 6px; }
.formula-row { display: grid; gap: 12px; grid-template-columns: repeat(auto-fit, minmax(min(240px, 100%), 1fr)); }
.formula-row .formula { margin: 8px 0; min-width: 0; }
.formula-row.stack { grid-template-columns: minmax(0, 1fr); }

/* Phương trình hoá học */
.equation {
  font-size: calc(1em * var(--fit, 1));
  background: var(--surface-2); border-radius: 8px; padding: 6px 14px; margin: 8px 0;
  overflow-x: auto; overflow-y: hidden; text-align: center;
}
.equation .katex-display { margin: .3em 0; }
.content .katex-display { overflow-x: auto; overflow-y: hidden; }
.katex-display.kfit > .katex { font-size: calc(1.21em * var(--kfit, 1)); }
.formula-body.is-scroll::after, .equation.is-scroll::after {
  content: "↔ vuốt ngang để xem hết"; display: block; position: sticky; left: 0;
  font-size: 12px; font-style: italic; color: var(--text-muted); text-align: left; padding-top: 2px;
}
.k-scroll { display: inline-block; max-width: 100%; overflow-x: auto; overflow-y: hidden; vertical-align: bottom; }

/* Ghi nhớ cuối bài */
.key-points {
  border-radius: 10px; padding: 12px 16px; margin: 18px 0 0;
  background: var(--accent-soft); border: 1px dashed var(--accent);
}
.key-points::before { content: "🎯 Ghi nhớ nhanh"; display: block; font-weight: 800; color: var(--accent); font-size: 13px; letter-spacing: .05em; text-transform: uppercase; margin-bottom: 2px; }

/* Bảng */
.table-wrap { overflow-x: auto; margin: 14px 0; border-radius: 10px; border: 1px solid var(--border); }
table { border-collapse: collapse; width: 100%; font-size: 15px; background: var(--surface); }
th, td { padding: 8px 12px; border-bottom: 1px solid var(--border); text-align: left; vertical-align: top; }
th { background: var(--accent-soft); color: var(--accent); font-weight: 700; white-space: nowrap; }
tr:last-child td { border-bottom: 0; }
td + td, th + th { border-left: 1px solid var(--border); }

/* Sơ đồ / hình minh hoạ */
.diagram { margin: 16px 0; text-align: center; }
.diagram svg { background: var(--surface-2); border-radius: 10px; height: auto; }
.diagram figcaption { font-size: 13.5px; color: var(--text-muted); margin-top: 6px; }
.diagram svg text { fill: var(--text); font-family: var(--font); }
.diagram .stroke { stroke: var(--text); }
.diagram .accent { stroke: var(--accent); fill: none; }
.diagram .accent-fill { fill: var(--accent); }

/* Sơ đồ bước (quy trình) */
.steps { list-style: none; padding: 0; counter-reset: step; display: grid; grid-template-columns: minmax(0, 1fr); gap: 8px; margin: 12px 0; }
.steps > li {
  counter-increment: step; position: relative; padding: 8px 12px 8px 46px;
  background: var(--surface-2); border-radius: 8px; margin: 0;
}
.steps > li::before {
  content: counter(step); position: absolute; left: 10px; top: 8px;
  width: 26px; height: 26px; border-radius: 50%; background: var(--accent); color: var(--on-accent);
  display: grid; place-items: center; font-weight: 700; font-size: 14px;
}
.flow { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; margin: 12px 0; }
.flow > span { background: var(--accent-soft); color: var(--accent); border-radius: 8px; padding: 5px 12px; font-weight: 600; }
.flow > i { font-style: normal; color: var(--text-muted); }

.two-col { display: grid; gap: 14px; grid-template-columns: repeat(auto-fit, minmax(min(260px, 100%), 1fr)); }
.two-col > .box { margin: 0; }

/* Nút lên đầu */
.to-top {
  position: fixed; right: 16px; bottom: 16px; z-index: 40; width: 44px; height: 44px; border-radius: 50%;
  border: 0; background: var(--accent, var(--brand)); color: var(--on-accent); font-size: 20px; cursor: pointer;
  box-shadow: var(--shadow); opacity: 0; pointer-events: none; transition: opacity .2s;
}
.to-top.show { opacity: 1; pointer-events: auto; }

/* ---------- Lịch sử: dòng thời gian · Địa lí: số liệu nổi bật ---------- */
.timeline { list-style: none; margin: 14px 0; padding: 0 0 0 4px; position: relative; }
.timeline::before {
  content: ""; position: absolute; left: 7px; top: 6px; bottom: 6px; width: 2px;
  background: color-mix(in srgb, var(--accent) 35%, transparent);
}
.timeline > li { position: relative; margin: 0 0 10px; padding: 0 0 0 26px; }
.timeline > li::before {
  content: ""; position: absolute; left: 2px; top: 8px; width: 12px; height: 12px; border-radius: 50%;
  background: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft);
}
.t-date {
  display: inline-block; font-weight: 800; color: var(--accent); background: var(--accent-soft);
  border-radius: 6px; padding: 1px 9px; margin-right: 8px; font-size: 14.5px; white-space: nowrap;
}
.timeline .t-note { display: block; color: var(--text-muted); font-size: 14.5px; }

.stats { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(160px, 100%), 1fr)); gap: 10px; margin: 14px 0; }
.stats > div {
  background: var(--surface-2); border: 1px solid var(--border); border-left: 4px solid var(--accent);
  border-radius: 10px; padding: 10px 12px;
}
.stats b { display: block; font-size: 22px; font-weight: 800; color: var(--accent); line-height: 1.2; }
.stats span { font-size: 14px; color: var(--text-muted); }

/* ---------- Ngữ văn: dàn ý · bài văn mẫu · thẻ tác phẩm ---------- */
.outline {
  border: 2px solid var(--outline); background: var(--outline-bg);
  border-radius: 12px; padding: 12px 18px 12px; margin: 16px 0;
}
.outline-name {
  display: inline-block; font-size: 12.5px; font-weight: 800; letter-spacing: .04em;
  color: var(--on-accent); background: var(--outline); padding: 2px 10px; border-radius: 6px; margin-bottom: 6px;
}
.outline > ol { margin: 4px 0 0; padding-left: 1.4em; }
.outline > ol > li { margin: 6px 0; }
.outline > ol > li > b:first-child { color: var(--outline); }
.outline ul { margin: 2px 0; }

.work-card { display: grid; grid-template-columns: max-content minmax(0, 1fr); gap: 4px 14px; margin: 12px 0;
  background: var(--surface-2); border-radius: 10px; padding: 12px 16px; border-left: 4px solid var(--accent); }
.work-card dt { font-weight: 700; color: var(--accent); }
.work-card dd { margin: 0; }

blockquote.quote {
  margin: 12px 0; padding: 8px 16px; border-left: 3px solid var(--accent); background: var(--surface-2);
  border-radius: 0 8px 8px 0; font-style: italic; white-space: pre-line;
}
blockquote.quote cite { display: block; font-style: normal; font-size: 13.5px; color: var(--text-muted); margin-top: 4px; }

.essay {
  background: var(--surface); border: 1px solid var(--border); border-radius: 12px;
  padding: 18px 22px; margin: 16px 0;
  font-family: "Times New Roman", "Noto Serif", "Liberation Serif", Tinos, serif;
  font-size: 17px; line-height: 1.8;
}
.essay p { text-indent: 1.6em; margin: .6em 0; text-align: justify; }
.essay p:has(br) { text-indent: 0; text-align: left; }  /* thơ: giữ dòng, không căn đều */
.essay .part {
  display: block; text-indent: 0; font-family: var(--font); font-size: 11.5px; font-weight: 800;
  letter-spacing: .08em; text-transform: uppercase; color: var(--accent); margin: 14px 0 -2px;
}
.essay .part:first-child { margin-top: 0; }
@media (max-width: 640px) {
  .essay { padding: 14px; font-size: 16px; }
  .work-card { grid-template-columns: minmax(0, 1fr); }
  .work-card dd { margin-bottom: 6px; }
}

/* ---------- Chuyển Lý thuyết / Bài tập ---------- */
.mode-switch {
  max-width: 1280px; margin: 10px auto 0; padding: 0 16px; display: flex; gap: 8px; flex-wrap: wrap;
}
.mode-switch a, .mode-switch span {
  display: inline-flex; align-items: center; gap: 6px; padding: 8px 16px; border-radius: 99px;
  border: 1px solid var(--border); background: var(--surface); color: var(--text);
  text-decoration: none; font-weight: 700; font-size: 15px;
}
.mode-switch a:hover { border-color: var(--brand); }
.mode-switch .current { background: var(--brand); border-color: var(--brand); color: var(--on-accent); }
.mode-switch .soon { opacity: .5; font-weight: 600; }
.lesson-ex-link {
  font-size: 13px; font-weight: 700; text-decoration: none; margin-left: auto; white-space: nowrap;
  padding: 3px 10px; border-radius: 99px; background: var(--accent-soft); color: var(--accent);
}

/* ---------- Bài tập ---------- */
.ex-toolbar {
  display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin-bottom: 18px;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 10px 14px;
}
.ex-toolbar .label { font-size: 13px; font-weight: 700; color: var(--text-muted); margin-right: 2px; }
.ex-toolbar .spacer { flex: 1; }
.exercise {
  --lv: var(--tip); --lv-bg: var(--tip-bg);
  border: 1px solid var(--border); border-left: 4px solid var(--lv); border-radius: 10px;
  background: var(--surface); padding: 12px 16px; margin: 12px 0;
}
.exercise[data-level="2"] { --lv: var(--def); --lv-bg: var(--def-bg); }
.exercise[data-level="3"] { --lv: var(--warn); --lv-bg: var(--warn-bg); }
.exercise[hidden] { display: none; }
.ex-head { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin-bottom: 6px; }
.ex-no { font-weight: 800; }
.exercise[data-level="3"] .ex-no::after { content: "*"; color: var(--warn); margin-left: 1px; }
.ex-level {
  font-size: 11.5px; font-weight: 800; letter-spacing: .04em; text-transform: uppercase;
  padding: 2px 8px; border-radius: 6px; background: var(--lv-bg); color: var(--lv);
}
.ex-src { font-size: 12.5px; color: var(--text-muted); margin-left: auto; }
.exercise ol.choices { list-style: upper-alpha; padding-left: 1.6em; margin: 6px 0; }
.exercise ol.choices.grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(200px, 100%), 1fr)); gap: 0 24px; }
details.solution { margin-top: 8px; border-top: 1px dashed var(--border); padding-top: 6px; }
details.solution > summary {
  cursor: pointer; font-weight: 700; color: var(--accent); font-size: 14.5px; list-style: none;
  display: inline-flex; align-items: center; gap: 6px;
}
details.solution > summary::-webkit-details-marker { display: none; }
details.solution > summary::before { content: "▸"; transition: transform .15s; }
details.solution[open] > summary::before { transform: rotate(90deg); }
details.solution[open] > summary { margin-bottom: 6px; }
.answer { font-weight: 700; background: var(--tip-bg); color: var(--text); border-radius: 8px; padding: 4px 10px; display: inline-block; margin-top: 4px; }
.answer::before { content: "✅ Đáp số: "; color: var(--tip); }

/* ---------- Ngoại ngữ (Tiếng Anh) ---------- */
.lesson-title .vi-title { font-size: 15px; font-weight: 500; color: var(--text-muted); }
.lesson-title .vi-title i { font-style: italic; }
.formula-body.pattern { font-size: 1.12em; font-weight: 600; line-height: 2; text-align: center; }
.pattern b { color: var(--formula); }
.pattern i {
  font-style: normal; font-weight: 650; white-space: nowrap;
  background: var(--surface); border: 1px solid var(--border); border-radius: 6px; padding: 1px 7px;
}
span.en { font-style: italic; }
.vi { color: var(--text-muted); }
span.en + .vi::before { content: " → "; font-style: normal; }
ul.examples { list-style: none; padding: 0; margin: 8px 0; }
ul.examples > li {
  margin: 5px 0; padding: 5px 12px; background: var(--surface-2);
  border-left: 3px solid var(--accent); border-radius: 0 8px 8px 0;
}
ul.examples .vi { display: block; font-size: 14.5px; }
ul.examples .vi::before { content: none; }
table.vocab td:first-child { font-weight: 650; }
.ipa { color: var(--text-muted); white-space: nowrap; font-family: "Segoe UI", "Lucida Sans Unicode", "Arial Unicode MS", var(--font); }
.pos { color: var(--accent); font-size: 12.5px; font-weight: 700; white-space: nowrap; }
.toc .vi-title { color: var(--text-muted); }
.toc .vi-title::before { content: "· "; }
.stress { font-weight: 800; color: var(--accent); text-decoration: underline 2px; text-underline-offset: 3px; }

/* Kí hiệu viết tắt: bảng đầu trang + chữ có chú thích */
.abbr-legend {
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 10px 16px; margin-bottom: 20px; box-shadow: var(--shadow);
}
.abbr-legend summary { font-weight: 700; cursor: pointer; }
.abbr-legend summary .muted { font-weight: 400; font-size: 14px; }
.abbr-legend dl { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(250px, 100%), 1fr)); gap: 4px 20px; margin: 10px 0 4px; }
.abbr-legend dl > div { display: flex; gap: 10px; align-items: baseline; font-size: 15px; }
.abbr-legend dt { flex: 0 0 76px; font-weight: 800; color: var(--accent); }
.abbr-legend dd { margin: 0; }
.ab { text-decoration: underline dotted 1.5px; text-underline-offset: 3px; cursor: help; }
.ab:focus { outline: none; background: var(--accent-soft); border-radius: 3px; }
.ab-tip {
  position: fixed; z-index: 100; max-width: min(320px, calc(100vw - 16px));
  background: var(--text); color: var(--bg); font-size: 13.5px; line-height: 1.45; font-style: normal; font-weight: 500;
  padding: 6px 10px; border-radius: 8px; box-shadow: var(--shadow); pointer-events: none;
}

/* ---------- Công thức nhanh (bảng tra) ---------- */
.formula-btn { display: inline-flex; align-items: center; gap: 5px; font-weight: 650; white-space: nowrap; }
.formula-btn b { color: var(--formula); font-size: 17px; line-height: 1; }

.fsheet {
  border: 0; padding: 0; margin: 0 0 0 auto;
  width: min(820px, 100vw); max-width: 100vw; height: 100vh; height: 100dvh; max-height: none;
  background: var(--bg); color: var(--text); box-shadow: -8px 0 30px rgba(0, 0, 0, .25);
}
.fsheet::backdrop { background: rgba(10, 14, 22, .5); }
.fsheet[open] { animation: fs-in .18s ease-out; }
@keyframes fs-in { from { transform: translateX(32px); opacity: 0; } }
.fsheet [hidden] { display: none !important; }
.fs-inner { display: flex; flex-direction: column; height: 100%; }
.fs-head { padding: 14px 16px 12px; background: var(--surface); border-bottom: 1px solid var(--border); }
.fs-head-row { display: flex; align-items: center; gap: 8px; }
.fs-head .spacer { flex: 1; }
.fs-head .icon-btn { white-space: nowrap; flex-shrink: 0; }
.fs-title { font-size: 20px; font-weight: 800; }
.fs-sub { font-size: 13px; color: var(--text-muted); }
.fs-controls { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin-top: 10px; }
.fs-controls input[type="search"] {
  flex: 1 1 240px; min-width: 0; padding: 8px 12px; border-radius: 8px; border: 1px solid var(--border);
  background: var(--surface-2); color: var(--text); font: inherit; font-size: 15px;
}
.fs-chips { display: flex; flex-wrap: wrap; gap: 6px; }
.chip {
  --c: var(--brand);
  border: 1px solid var(--border); background: var(--surface); color: var(--text);
  border-radius: 99px; padding: 5px 12px; font: inherit; font-size: 14px; font-weight: 600; cursor: pointer;
}
.chip:hover { border-color: var(--c); }
.chip[aria-pressed="true"] { background: var(--c); border-color: var(--c); color: var(--on-accent); }
.fs-check { display: inline-flex; align-items: center; gap: 6px; font-size: 14px; cursor: pointer; }

.fs-body { flex: 1; overflow: auto; padding: 4px 16px 40px; }
.fs-group-title {
  font-size: 13px; font-weight: 800; text-transform: uppercase; letter-spacing: .08em;
  color: var(--accent); border-bottom: 2px solid var(--accent); padding: 18px 0 4px; margin-bottom: 4px;
}
.fs-lesson { margin: 14px 0 0; }
.fs-lesson-title {
  display: flex; align-items: baseline; gap: 8px; flex-wrap: wrap;
  font-weight: 700; font-size: 15.5px; color: var(--text); text-decoration: none;
}
.fs-lesson-title .lesson-no { font-size: 12px; padding: 2px 8px; }
.fs-go { font-size: 12.5px; font-weight: 600; color: var(--accent); opacity: 0; transition: opacity .15s; }
.fs-lesson-title:hover .fs-go, .fs-lesson-title:focus-visible .fs-go { opacity: 1; }
.fs-items { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(320px, 100%), 1fr)); gap: 8px; margin-top: 6px; }
.fsheet .formula { margin: 0; padding: 8px 12px 10px; border-width: 1.5px; min-width: 0; }
.fsheet .formula-name { font-size: 12px; }
.fsheet .formula-body { font-size: calc(1.1em * var(--fit, 1)); }
.fsheet .formula-legend { grid-template-columns: 1fr; font-size: 13px; gap: 0; padding-top: 6px; }
.fsheet .formula-note { font-size: 12.5px; }
.fsheet .timeline, .fsheet .stats, .fsheet .outline { grid-column: 1 / -1; margin: 2px 0; }
.fsheet .equation { grid-column: 1 / -1; margin: 0; padding: 0 12px; font-size: calc(14.5px * var(--fit, 1)); }
.fsheet .equation .katex-display { margin: .25em 0; }
.fs-empty { text-align: center; color: var(--text-muted); padding: 48px 0; }

@media (max-width: 640px) {
  .fs-items { grid-template-columns: minmax(0, 1fr); }
  .fs-go { opacity: 1; }
}

.footer { text-align: center; color: var(--text-muted); font-size: 13px; padding: 24px 16px 40px; }

/* ---------- Điện thoại / máy tính bảng ---------- */
@media (max-width: 900px) {
  .layout { grid-template-columns: minmax(0, 1fr); gap: 12px; }
  .toc { position: static; max-height: none; }
  .toc-toggle { display: block; width: 100%; text-align: left; }
  .toc:not(.open) .toc-body { display: none; }
  .toc.open .toc-body { max-height: 60vh; overflow: auto; margin-top: 8px; }
  .toc-title { display: none; }
  .lesson { padding: 16px; }
  body { font-size: 16px; }
  .hide-sm { display: none; }
  .ipa { white-space: normal; }
  .t-date { white-space: normal; }
}

/* ---------- In ấn ---------- */
@media print {
  .topbar, .tabs, .toc, .to-top { display: none !important; }
  .panel[hidden] { display: block !important; }
  .layout { display: block; padding: 0; }
  .lesson { box-shadow: none; break-inside: avoid-page; }
  .formula, .box { break-inside: avoid; }
  body { background: #fff; }

  /* In riêng bảng công thức nhanh */
  html.print-formulas body > :not(.fsheet) { display: none !important; }
  html.print-formulas .fsheet {
    position: static; width: auto; height: auto; max-width: none; margin: 0;
    box-shadow: none; background: #fff; animation: none;
  }
  html.print-formulas .fs-inner { display: block; height: auto; }
  html.print-formulas .fs-body { overflow: visible; padding: 0; }
  html.print-formulas .fs-controls, html.print-formulas .fs-head .icon-btn, html.print-formulas .fs-go { display: none !important; }
  .fsheet .formula, .fsheet .equation { break-inside: avoid; }
  .fs-lesson-title { break-after: avoid-page; }
}