/* ============================================================
   mathify.us — SSC Mathematics Shared Stylesheet
   ============================================================ */

/* Google Fonts */
@import url('https://fonts.googleapis.com/css2?family=Playfair+Display:wght@400;700&family=JetBrains+Mono:wght@400;700&family=Inter:wght@400;500;600;700&display=swap');

/* ── Design Tokens ───────────────────────────────────────── */
:root {
  --bg:     #0d1117;
  --bg2:    #161b22;
  --bg3:    #21262d;
  --border: rgba(255,255,255,0.08);
  --ink:    #e6edf3;
  --dim:    #8b949e;
  --muted:  #6e7681;
  --blue:   #4f8ef7;
  --sky:    #38c8f5;
  --mint:   #2fe0b4;
  --gold:   #f0b429;
  --violet: #9b7fff;
  --r:      6px;
}

/* ── Reset & Base ────────────────────────────────────────── */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  font-size: 16px;
}

body {
  background-color: var(--bg);
  color: var(--ink);
  font-family: 'Inter', system-ui, -apple-system, sans-serif;
  font-size: 1rem;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

a {
  color: var(--blue);
  text-decoration: none;
  transition: color 0.15s ease;
}

a:hover {
  color: var(--sky);
}

img, svg {
  display: block;
  max-width: 100%;
}

ul, ol {
  list-style: none;
}

/* ── Topbar ──────────────────────────────────────────────── */
.topbar {
  position: sticky;
  top: 0;
  z-index: 100;
  height: 52px;
  background: var(--bg);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  padding: 0 24px;
  gap: 32px;
}

/* ── Logo ────────────────────────────────────────────────── */
.logo {
  font-family: 'JetBrains Mono', monospace;
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--blue);
  letter-spacing: -0.02em;
  display: flex;
  align-items: center;
  gap: 0;
  white-space: nowrap;
  flex-shrink: 0;
}

.logo em {
  font-style: italic;
  color: var(--sky);
}

.logo-tag {
  display: inline-flex;
  align-items: center;
  margin-left: 8px;
  padding: 2px 7px;
  font-family: 'Inter', sans-serif;
  font-size: 0.6rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--mint);
  background: rgba(47, 224, 180, 0.12);
  border: 1px solid rgba(47, 224, 180, 0.25);
  border-radius: 99px;
  line-height: 1;
}

/* ── Nav Links ───────────────────────────────────────────── */
.nav-links {
  display: flex;
  align-items: center;
  gap: 4px;
  flex: 1;
}

.nl {
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--dim);
  padding: 6px 10px;
  border-radius: var(--r);
  transition: color 0.15s ease, background 0.15s ease;
  white-space: nowrap;
}

.nl:hover {
  color: var(--ink);
  background: var(--bg3);
}

.nl.active {
  color: var(--blue);
  background: rgba(79, 142, 247, 0.1);
}

/* ── Hero ────────────────────────────────────────────────── */
.hero {
  text-align: center;
  padding: 72px 24px 56px;
  max-width: 760px;
  margin: 0 auto;
}

.hero h1 {
  font-family: 'Playfair Display', Georgia, serif;
  font-size: clamp(2rem, 5vw, 3.25rem);
  font-weight: 700;
  color: var(--ink);
  line-height: 1.2;
  letter-spacing: -0.02em;
  margin-bottom: 14px;
}

.hero-sub {
  font-size: 1rem;
  color: var(--dim);
  margin-bottom: 24px;
  font-weight: 400;
}

/* ── Hero Chips ──────────────────────────────────────────── */
.hero-chips {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 8px;
  margin-bottom: 20px;
}

.chip {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 4px 12px;
  font-size: 0.78rem;
  font-weight: 500;
  border-radius: 99px;
  border: 1px solid var(--border);
  background: var(--bg2);
  color: var(--dim);
  white-space: nowrap;
}

.chip.blue   { color: var(--blue);   background: rgba(79,142,247,0.1);   border-color: rgba(79,142,247,0.25); }
.chip.sky    { color: var(--sky);    background: rgba(56,200,245,0.1);   border-color: rgba(56,200,245,0.25); }
.chip.mint   { color: var(--mint);   background: rgba(47,224,180,0.1);   border-color: rgba(47,224,180,0.25); }
.chip.gold   { color: var(--gold);   background: rgba(240,180,41,0.1);   border-color: rgba(240,180,41,0.25); }
.chip.violet { color: var(--violet); background: rgba(155,127,255,0.1);  border-color: rgba(155,127,255,0.25); }

/* ── Breadcrumb ──────────────────────────────────────────── */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: 6px;
  justify-content: center;
  font-size: 0.78rem;
  color: var(--muted);
  margin-top: 4px;
}

.breadcrumb a {
  color: var(--muted);
}

.breadcrumb a:hover {
  color: var(--dim);
}

.breadcrumb .sep {
  color: var(--muted);
  opacity: 0.5;
}

/* ── Main Layout ─────────────────────────────────────────── */
.main {
  display: grid;
  grid-template-columns: 240px 1fr;
  grid-template-areas: "sidebar content";
  gap: 0;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px 64px;
  align-items: start;
}

/* ── Sidebar ─────────────────────────────────────────────── */
.sidebar {
  grid-area: sidebar;
  position: sticky;
  top: 68px;
  padding: 24px 16px 24px 0;
  max-height: calc(100vh - 80px);
  overflow-y: auto;
}

.sidebar::-webkit-scrollbar { width: 4px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }
.sidebar::-webkit-scrollbar-thumb { background: var(--bg3); border-radius: 2px; }

/* ── Sticky Table of Contents ────────────────────────────── */
.stoc {
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.stoc-heading {
  font-size: 0.7rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  padding: 0 8px 10px;
}

.stoc a {
  display: block;
  font-size: 0.82rem;
  color: var(--dim);
  padding: 5px 8px;
  border-radius: var(--r);
  transition: color 0.15s, background 0.15s;
  line-height: 1.4;
}

.stoc a:hover {
  color: var(--ink);
  background: var(--bg3);
}

.stoc a.active {
  color: var(--blue);
  background: rgba(79,142,247,0.1);
}

/* ── Content ─────────────────────────────────────────────── */
.content {
  grid-area: content;
  min-width: 0;
  padding: 24px 0 24px 32px;
  border-left: 1px solid var(--border);
}

/* ── Topic Section ───────────────────────────────────────── */
.topic {
  padding: 40px 0;
  border-bottom: 1px solid var(--border);
}

.topic:last-child {
  border-bottom: none;
}

.topic h2 {
  font-family: 'Playfair Display', Georgia, serif;
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--ink);
  margin-bottom: 6px;
  letter-spacing: -0.01em;
}

.topic h3 {
  font-size: 1rem;
  font-weight: 600;
  color: var(--dim);
  margin: 24px 0 10px;
}

.topic p {
  color: var(--dim);
  font-size: 0.95rem;
  margin-bottom: 12px;
  max-width: 68ch;
}

/* ── Analogy Block ───────────────────────────────────────── */
.analogy {
  border-left: 3px solid var(--mint);
  background: var(--bg2);
  border-radius: 0 var(--r) var(--r) 0;
  padding: 16px 20px;
  margin: 20px 0;
  color: var(--dim);
  font-size: 0.9rem;
  line-height: 1.7;
}

.analogy strong {
  color: var(--mint);
  font-weight: 600;
}

/* ── Formula Card ────────────────────────────────────────── */
.formula-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 18px 20px;
  margin: 16px 0;
}

.fc-title {
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 10px;
}

.fc-eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 1rem;
  color: var(--gold);
  background: var(--bg3);
  border-radius: 4px;
  padding: 10px 14px;
  margin: 8px 0;
  overflow-x: auto;
}

.fc-note {
  font-size: 0.82rem;
  color: var(--muted);
  margin-top: 8px;
  font-style: italic;
}

/* ── Problem ─────────────────────────────────────────────── */
.problem {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  margin: 20px 0;
  overflow: hidden;
}

.ph {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 12px 16px;
  border-bottom: 1px solid var(--border);
  flex-wrap: wrap;
}

.pb {
  padding: 16px;
  font-size: 0.92rem;
  color: var(--dim);
  line-height: 1.7;
}

/* ── Board Tag ───────────────────────────────────────────── */
.board-tag {
  display: inline-flex;
  align-items: center;
  padding: 3px 9px;
  font-size: 0.7rem;
  font-weight: 600;
  letter-spacing: 0.04em;
  border-radius: 99px;
  background: rgba(155,127,255,0.12);
  color: var(--violet);
  border: 1px solid rgba(155,127,255,0.2);
}

/* ── Solution Toggle ─────────────────────────────────────── */
.sol-toggle {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 10px 16px;
  background: rgba(79,142,247,0.06);
  border: none;
  border-top: 1px solid var(--border);
  width: 100%;
  text-align: left;
  cursor: pointer;
  font-family: 'Inter', sans-serif;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--blue);
  letter-spacing: 0.02em;
  transition: background 0.15s ease;
}

.sol-toggle:hover {
  background: rgba(79,142,247,0.12);
}

.sol-toggle .arrow {
  display: inline-block;
  transition: transform 0.2s ease;
  font-style: normal;
}

.sol-toggle.open .arrow {
  transform: rotate(90deg);
}

/* ── Solution Panel ──────────────────────────────────────── */
.solution {
  display: none;
  padding: 16px;
  border-top: 1px solid var(--border);
  background: rgba(13,17,23,0.5);
}

.solution.open {
  display: block;
}

/* ── Step Solution ───────────────────────────────────────── */
.ss {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.ss > div {
  display: grid;
  grid-template-columns: 28px 1fr;
  gap: 10px;
  align-items: start;
}

.sn {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--mint);
  background: rgba(47,224,180,0.1);
  border-radius: 4px;
  width: 28px;
  height: 28px;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.sc {
  font-size: 0.88rem;
  color: var(--dim);
  line-height: 1.7;
  padding-top: 3px;
}

.se {
  display: inline-block;
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.85rem;
  color: var(--gold);
  background: rgba(240,180,41,0.08);
  border-radius: 4px;
  padding: 1px 6px;
}

.sr2 {
  font-weight: 700;
  color: var(--mint);
}

/* ── Simulator ───────────────────────────────────────────── */
.sim-box {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 24px;
  margin: 20px 0;
}

.sim-title {
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 16px;
}

/* ── Chapter Nav ─────────────────────────────────────────── */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 20px 0;
  border-top: 1px solid var(--border);
  margin-top: 32px;
  gap: 12px;
}

.chapter-nav a {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 10px 16px;
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  font-size: 0.85rem;
  font-weight: 500;
  color: var(--dim);
  transition: color 0.15s, border-color 0.15s, background 0.15s;
}

.chapter-nav a:hover {
  color: var(--ink);
  border-color: rgba(255,255,255,0.18);
  background: var(--bg3);
}

/* ── Board Cell ──────────────────────────────────────────── */
.board-cell {
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 8px 6px;
  border-radius: var(--r);
  font-size: 0.78rem;
  font-weight: 500;
  color: var(--muted);
  background: var(--bg3);
  border: 1px solid var(--border);
  text-align: center;
  transition: all 0.15s ease;
  min-height: 38px;
}

.board-cell.available {
  color: var(--blue);
  background: rgba(79,142,247,0.07);
  border-color: rgba(79,142,247,0.2);
  cursor: pointer;
}

.board-cell.available:hover {
  background: rgba(79,142,247,0.15);
  border-color: rgba(79,142,247,0.4);
  color: var(--sky);
  transform: translateY(-1px);
  box-shadow: 0 4px 12px rgba(79,142,247,0.15);
}

/* ── Q-Card (Board Paper Question) ──────────────────────── */
.q-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  margin: 16px 0;
  overflow: hidden;
}

.q-header {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
  background: rgba(255,255,255,0.02);
  flex-wrap: wrap;
}

.q-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  font-weight: 700;
  color: var(--muted);
  background: var(--bg3);
  padding: 2px 8px;
  border-radius: 4px;
  flex-shrink: 0;
}

.q-marks {
  font-size: 0.72rem;
  font-weight: 600;
  color: var(--gold);
  background: rgba(240,180,41,0.1);
  border: 1px solid rgba(240,180,41,0.2);
  padding: 2px 8px;
  border-radius: 99px;
  margin-left: auto;
  flex-shrink: 0;
}

.q-chapter-tag {
  font-size: 0.7rem;
  font-weight: 500;
  color: var(--violet);
  background: rgba(155,127,255,0.1);
  border: 1px solid rgba(155,127,255,0.2);
  padding: 2px 8px;
  border-radius: 99px;
}

/* ── Standalone page containers ──────────────────────────── */
.page-wrap {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px 80px;
}

.section-header {
  padding: 48px 0 28px;
}

.section-header h2 {
  font-family: 'Playfair Display', Georgia, serif;
  font-size: 1.8rem;
  font-weight: 700;
  color: var(--ink);
  letter-spacing: -0.01em;
  margin-bottom: 6px;
}

.section-header p {
  color: var(--dim);
  font-size: 0.9rem;
}

/* ── Chapter Cards ───────────────────────────────────────── */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 14px;
  margin-bottom: 32px;
}

.ch-card {
  display: flex;
  flex-direction: column;
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 18px;
  transition: border-color 0.15s, transform 0.15s, box-shadow 0.15s;
  text-decoration: none;
  color: inherit;
}

.ch-card:hover {
  border-color: rgba(79,142,247,0.35);
  transform: translateY(-2px);
  box-shadow: 0 6px 20px rgba(0,0,0,0.3);
  color: inherit;
}

.ch-badge {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.7rem;
  font-weight: 700;
  color: var(--muted);
  margin-bottom: 10px;
}

.ch-title {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--ink);
  margin-bottom: 6px;
  line-height: 1.3;
}

.ch-desc {
  font-size: 0.82rem;
  color: var(--dim);
  flex: 1;
  margin-bottom: 12px;
  line-height: 1.5;
}

.ch-chips {
  display: flex;
  flex-wrap: wrap;
  gap: 5px;
}

.ch-chip {
  font-size: 0.68rem;
  font-weight: 500;
  padding: 2px 8px;
  border-radius: 99px;
  background: var(--bg3);
  color: var(--muted);
  border: 1px solid var(--border);
}

/* ── Group Labels ────────────────────────────────────────── */
.group-label {
  display: flex;
  align-items: center;
  gap: 10px;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  margin: 32px 0 14px;
  color: var(--muted);
}

.group-label::after {
  content: '';
  flex: 1;
  height: 1px;
  background: var(--border);
}

.group-label.algebra  { color: var(--blue); }
.group-label.geometry { color: var(--mint); }
.group-label.trig     { color: var(--gold); }
.group-label.stats    { color: var(--violet); }

/* ── How It Works ────────────────────────────────────────── */
.how-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 16px;
  margin: 24px 0;
}

.how-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 24px 20px;
  text-align: center;
}

.how-icon {
  font-size: 2rem;
  margin-bottom: 12px;
  display: block;
}

.how-step {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.68rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 6px;
}

.how-title {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--ink);
  margin-bottom: 6px;
}

.how-desc {
  font-size: 0.82rem;
  color: var(--muted);
  line-height: 1.5;
}

.how-arrow {
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--border);
  font-size: 1.4rem;
}

/* ── Board Grid Table ────────────────────────────────────── */
.board-grid-wrap {
  overflow-x: auto;
  margin: 0 0 32px;
}

.board-table {
  width: 100%;
  border-collapse: separate;
  border-spacing: 5px;
  min-width: 600px;
}

.board-table th {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.72rem;
  font-weight: 700;
  color: var(--muted);
  padding: 6px 8px;
  text-align: center;
  letter-spacing: 0.05em;
}

.board-table td.row-label {
  font-size: 0.78rem;
  font-weight: 600;
  color: var(--dim);
  padding: 4px 10px 4px 0;
  white-space: nowrap;
  text-align: right;
}

/* ── Footer ──────────────────────────────────────────────── */
.footer {
  border-top: 1px solid var(--border);
  padding: 32px 24px;
  text-align: center;
  color: var(--muted);
  font-size: 0.82rem;
}

.footer a {
  color: var(--muted);
}

.footer a:hover {
  color: var(--dim);
}

/* ── Responsive ──────────────────────────────────────────── */
@media (max-width: 860px) {
  .topbar {
    padding: 0 16px;
    gap: 16px;
  }

  .nav-links {
    display: none;
  }

  .hero {
    padding: 48px 16px 40px;
  }

  .main {
    grid-template-columns: 1fr;
    grid-template-areas: "content";
    padding: 0 16px 48px;
  }

  .sidebar {
    display: none;
  }

  .content {
    padding: 16px 0;
    border-left: none;
  }

  .chapter-grid {
    grid-template-columns: 1fr;
  }

  .how-grid {
    grid-template-columns: 1fr;
  }

  .how-arrow {
    transform: rotate(90deg);
  }

  .chapter-nav {
    flex-direction: column;
  }

  .chapter-nav a {
    width: 100%;
    justify-content: center;
  }

  .page-wrap {
    padding: 0 16px 48px;
  }
}

@media (max-width: 560px) {
  .hero h1 {
    font-size: 1.75rem;
  }

  .board-tag {
    font-size: 0.65rem;
  }

  .ph {
    gap: 6px;
  }
}

/* ── Pedagogical Components ──────────────────────────────── */

/* Why This Matters banner */
.why-banner {
  display: flex;
  gap: 20px;
  align-items: flex-start;
  background: linear-gradient(135deg, rgba(47,224,180,0.06) 0%, rgba(79,142,247,0.06) 100%);
  border: 1px solid rgba(47,224,180,0.2);
  border-radius: var(--r);
  padding: 24px;
  margin-bottom: 8px;
}

.why-icon {
  font-size: 2.2rem;
  flex-shrink: 0;
  margin-top: 2px;
}

.why-banner h2 {
  font-family: 'Playfair Display', Georgia, serif;
  font-size: 1.35rem !important;
  font-weight: 700;
  color: var(--mint) !important;
  margin-bottom: 8px !important;
}

.why-banner p {
  color: var(--dim);
  font-size: 0.95rem;
  line-height: 1.7;
  margin-bottom: 0 !important;
  max-width: 100% !important;
}

/* Concept block — intuition-first layout */
.concept-block {
  border-left: 3px solid var(--blue);
  background: var(--bg2);
  border-radius: 0 var(--r) var(--r) 0;
  padding: 20px 22px;
  margin: 20px 0;
}

.concept-block h3 {
  font-size: 1rem;
  font-weight: 700;
  color: var(--ink) !important;
  margin: 0 0 6px !important;
}

.concept-real-world {
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--gold);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 10px;
}

.concept-why {
  margin-top: 12px;
  padding: 10px 14px;
  background: rgba(155,127,255,0.07);
  border-left: 2px solid var(--violet);
  border-radius: 0 4px 4px 0;
  font-size: 0.84rem;
  color: var(--dim);
  font-style: italic;
}

.concept-why strong {
  color: var(--violet);
  font-style: normal;
}

/* Mistake cards */
.mistake-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 14px;
  margin: 16px 0;
}

.mistake-card {
  background: var(--bg2);
  border: 1px solid rgba(240,80,80,0.2);
  border-radius: var(--r);
  padding: 16px 18px;
}

.mistake-card .wrong {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  color: #f07070;
  background: rgba(240,80,80,0.08);
  border-radius: 4px;
  padding: 4px 8px;
  margin: 6px 0;
  display: block;
}

.mistake-card .right {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.82rem;
  color: var(--mint);
  background: rgba(47,224,180,0.08);
  border-radius: 4px;
  padding: 4px 8px;
  margin: 6px 0;
  display: block;
}

.mistake-label {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  margin-bottom: 6px;
}

.mistake-label.bad  { color: #f07070; }
.mistake-label.good { color: var(--mint); }

/* Insight / aha-moment box */
.insight {
  border-left: 3px solid var(--gold);
  background: rgba(240,180,41,0.06);
  border-radius: 0 var(--r) var(--r) 0;
  padding: 16px 20px;
  margin: 20px 0;
  font-size: 0.9rem;
  color: var(--dim);
  line-height: 1.7;
}

.insight strong {
  color: var(--gold);
  font-weight: 600;
}

/* Step-by-step build */
.build-steps {
  counter-reset: bstep;
  margin: 16px 0;
}

.build-step {
  display: grid;
  grid-template-columns: 36px 1fr;
  gap: 14px;
  align-items: start;
  margin-bottom: 14px;
}

.build-step::before {
  counter-increment: bstep;
  content: counter(bstep);
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--blue);
  background: rgba(79,142,247,0.12);
  width: 36px;
  height: 36px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.build-step-content {
  padding-top: 6px;
  font-size: 0.92rem;
  color: var(--dim);
  line-height: 1.7;
}

/* Visual Diagram areas */
.visual-box {
  background: var(--bg3);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 16px;
  margin: 16px 0;
  text-align: center;
}

.visual-label {
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 12px;
}

/* Number set hierarchy */
.set-hierarchy {
  display: flex;
  flex-direction: column;
  gap: 6px;
  margin: 16px 0;
}

.set-row {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 14px;
  border-radius: var(--r);
  font-size: 0.88rem;
}

.set-row .set-name {
  font-weight: 600;
  min-width: 120px;
}

.set-row .set-example {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.8rem;
  color: var(--muted);
}

/* Formula card with derivation hint */
.formula-card .fc-derivation {
  margin-top: 10px;
  padding-top: 10px;
  border-top: 1px solid var(--border);
  font-size: 0.8rem;
  color: var(--muted);
  font-style: italic;
}

/* Board question topic tags */
.q-marks {
  font-size: 0.72rem;
}

/* Quick ref grid */
.qref-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
  gap: 10px;
  margin: 16px 0;
}

.qref-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 12px 14px;
}

.qref-title {
  font-size: 0.7rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 6px;
}

.qref-eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.88rem;
  color: var(--gold);
}

@media (max-width: 768px) {
  .why-banner { flex-direction: column; gap: 12px; }
  .mistake-grid { grid-template-columns: 1fr; }
  .qref-grid { grid-template-columns: 1fr 1fr; }
}

/* ── Bilingual Toggle ────────────────────────────────────── */
.lang-bn { display: none; }

body.bn .lang-en { display: none !important; }
body.bn .lang-bn { display: revert !important; }

body.bn div.lang-bn,
body.bn section.lang-bn,
body.bn p.lang-bn { display: block !important; }

.lang-toggle-btn {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 5px 12px;
  border: 1px solid var(--border);
  border-radius: 99px;
  background: var(--bg2);
  color: var(--dim);
  font-family: 'Inter', sans-serif;
  font-size: 0.78rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.15s ease;
  margin-left: auto;
  white-space: nowrap;
  flex-shrink: 0;
}

.lang-toggle-btn:hover {
  color: var(--ink);
  border-color: rgba(255,255,255,0.18);
  background: var(--bg3);
}

body.bn .lang-toggle-btn {
  color: var(--gold);
  border-color: rgba(240,180,41,0.3);
  background: rgba(240,180,41,0.07);
}

/* ── KaTeX Dark Theme Overrides ─────────────────────────── */
.katex { font-size: 1em; color: var(--ink); }
.katex-display { margin: 1rem 0; overflow-x: auto; }
.katex-display > .katex { color: var(--gold); font-size: 1.1em; }

/* Inline KaTeX inside .fc-eq gets larger */
.fc-eq .katex { font-size: 1.05em; color: var(--gold); }

/* KaTeX inside .se (inline equation spans) */
.se .katex { font-size: 0.95em; }

/* KaTeX display blocks inside formula-cards */
.formula-card .katex-display > .katex { color: var(--gold); }

/* Ensure KaTeX doesn't overflow narrow containers */
.katex-html { overflow-x: auto; }

/* ── Quick Reference Grid ────────────────────────────────── */
.qref-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 12px;
  margin-top: 12px;
}
.qref-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--r);
  padding: 14px 16px;
}
.qref-label {
  font-size: 0.68rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: .06em;
  color: var(--violet);
  margin-bottom: 6px;
}
.qref-eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.88rem;
  color: var(--gold);
}