/* ============================================================
   Interactive Textbook — Shared Design Tokens & Components
   ============================================================ */

:root {
  --color-void: #000000;
  --color-fog-border: #e5e7eb;
  --color-chalk: #f1f1f1;
  --color-graphite: #666666;
  --color-ash: #8f8f8f;
  --color-canvas: #ffffff;
  --font-inter: 'Inter', ui-sans-serif, system-ui, -apple-system, sans-serif;
  --text-caption: 13px;
  --text-body: 16px;
  --text-heading: 22px;
  --text-heading-lg: 28px;
  --radius-cards: 6.08px;
  --radius-full: 9999px;
  --radius-softbutton: 40px;
  --spacing-4: 4px;
  --spacing-8: 8px;
  --spacing-12: 12px;
  --spacing-16: 16px;
  --spacing-20: 20px;
  --spacing-24: 24px;
  --spacing-32: 32px;
  --spacing-40: 40px;
  --spacing-52: 52px;
  --spacing-64: 64px;
  --spacing-80: 80px;
  --spacing-112: 112px;
}

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--font-inter);
  background: var(--color-canvas);
  color: var(--color-void);
  font-size: var(--text-body);
  line-height: 1.6;
  font-feature-settings: "calt","liga";
  -webkit-font-smoothing: antialiased;
}

/* NAV */
nav {
  position: sticky; top: 0; z-index: 100;
  height: 64px; background: var(--color-canvas);
  border-bottom: 1px solid var(--color-fog-border);
  display: flex; align-items: center;
  padding: 0 var(--spacing-40);
}
.nav-inner {
  max-width: 1200px; width: 100%; margin: 0 auto;
  display: flex; align-items: center; justify-content: space-between;
}
.nav-back { font-size:18px; color:var(--color-graphite); text-decoration:none; margin-right:16px; transition:color 0.15s; }
.nav-back:hover { color:var(--color-void); }
.nav-brand { font-size: 14px; font-weight: 600; color: var(--color-void); text-decoration: none; letter-spacing: -0.01em; }
.nav-links { display: flex; gap: var(--spacing-24); list-style: none; }
.nav-links a { font-size: 14px; font-weight: 500; color: var(--color-void); text-decoration: none; transition: color 0.15s; }
.nav-links a:hover { color: var(--color-graphite); }
.nav-badge { font-size: 13px; font-weight: 500; color: var(--color-graphite); background: rgba(0,0,0,0.04); border-radius: var(--radius-softbutton); padding: 6px 14px; }

/* LANG TOGGLE */
.lang-toggle { display: flex; gap: 4px; }
.lang-btn {
  font-size: 12px; font-weight: 600; font-family: var(--font-inter);
  padding: 4px 10px; border-radius: var(--radius-softbutton);
  border: 1px solid var(--color-fog-border); background: none;
  color: var(--color-ash); cursor: pointer; transition: all 0.15s;
}
.lang-btn.active { background: var(--color-void); color: #fff; border-color: var(--color-void); }

/* PAGE */
.page { max-width: 1200px; margin: 0 auto; padding: 0 var(--spacing-40); }

/* HERO */
.hero { padding: var(--spacing-112) 0 var(--spacing-80); text-align: center; border-bottom: 1px solid var(--color-fog-border); }
.hero-eyebrow { font-size: var(--text-caption); font-weight: 500; color: var(--color-graphite); letter-spacing: 0.011em; text-transform: uppercase; margin-bottom: var(--spacing-20); }
.hero-title { font-size: clamp(36px,5vw,56px); font-weight: 600; color: var(--color-void); letter-spacing: -1.44px; line-height: 1.08; margin-bottom: var(--spacing-20); }
.hero-subtitle { font-size: 18px; font-weight: 400; color: var(--color-graphite); line-height: 1.5; max-width: 520px; margin: 0 auto var(--spacing-40); }
.hero-chips { display: flex; gap: var(--spacing-8); justify-content: center; flex-wrap: wrap; }
.chip { font-size: 13px; font-weight: 500; color: var(--color-void); background: rgba(0,0,0,0.04); border-radius: var(--radius-softbutton); padding: 7px 16px; border: none; cursor: default; letter-spacing: 0.011em; }

/* SECTION */
.section { padding: var(--spacing-80) 0; border-bottom: 1px solid var(--color-fog-border); opacity: 0; transform: translateY(24px); transition: opacity 0.5s ease, transform 0.5s ease; }
.section.visible { opacity: 1; transform: none; }
@media print { .section { opacity: 1 !important; transform: none !important; } }
.section-label { font-size: var(--text-caption); font-weight: 500; color: var(--color-graphite); letter-spacing: 0.011em; text-transform: uppercase; margin-bottom: var(--spacing-12); }
.section-title { font-size: var(--text-heading-lg); font-weight: 600; color: var(--color-void); line-height: 1.21; margin-bottom: var(--spacing-24); }
.section-body { font-size: 16px; color: var(--color-graphite); line-height: 1.65; max-width: 680px; margin-bottom: var(--spacing-32); }
.section-body p { margin-bottom: var(--spacing-12); }

/* TWO-COL */
.two-col { display: grid; grid-template-columns: 1fr 1fr; gap: var(--spacing-64); align-items: start; }
.two-col.reverse { direction: rtl; }
.two-col.reverse > * { direction: ltr; }

/* EQUATION */
.eq-block {
  display: flex; align-items: center; justify-content: flex-start; gap: var(--spacing-16);
  margin: var(--spacing-20) 0;
  padding: var(--spacing-12) 0;
  overflow-x: auto;
  counter-increment: eq-counter;
}
.eq-block .katex-display { margin: 0 !important; font-size: 1.1em; flex: 0 1 auto; }
.eq-block::after {
  content: "(" counter(eq-counter) ")";
  font-size: 14px; font-weight: 500; color: var(--color-graphite);
  margin-left: auto; flex-shrink: 0; padding-right: var(--spacing-8);
  font-variant-numeric: tabular-nums;
}
.section { counter-reset: eq-counter; }

/* CONCEPT LIST */
.concept-list { list-style: none; margin-top: var(--spacing-16); }
.concept-list li { font-size: 15px; color: var(--color-graphite); line-height: 1.6; padding: var(--spacing-12) 0; border-bottom: 1px solid var(--color-fog-border); display: flex; gap: var(--spacing-12); align-items: flex-start; }
.concept-list li:last-child { border-bottom: none; }
.concept-list li::before { content: "—"; color: var(--color-ash); flex-shrink: 0; margin-top: 1px; }

/* FIGURE CARD */
.figure-card { border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); overflow: hidden; }
.figure-header { padding: var(--spacing-20) var(--spacing-24); border-bottom: 1px solid var(--color-fog-border); display: flex; align-items: center; justify-content: space-between; gap: 8px; flex-wrap: wrap; }
.figure-label { font-size: var(--text-caption); font-weight: 500; color: var(--color-graphite); letter-spacing: 0.011em; text-transform: uppercase; }
.figure-title { font-size: 14px; font-weight: 500; color: var(--color-void); }
.figure-body { padding: var(--spacing-24); }
.figure-full { border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); overflow: hidden; margin-top: var(--spacing-32); }

/* PLOTLY CONTAINER */
.plotly-container { width: 100%; min-height: 380px; border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); overflow: hidden; }
.modebar { opacity: 0.3; transition: opacity 0.2s; }
.modebar:hover { opacity: 1; }

/* SLIDERS */
.controls { margin-top: var(--spacing-20); display: flex; flex-direction: column; gap: var(--spacing-16); }
.control-row { display: flex; align-items: center; gap: var(--spacing-16); }
.control-label { font-size: 13px; font-weight: 500; color: var(--color-void); min-width: 120px; white-space: nowrap; }
.control-label sub { font-size: 10px; }
input[type="range"] { -webkit-appearance: none; appearance: none; flex: 1; height: 2px; background: var(--color-fog-border); border-radius: 1px; outline: none; cursor: pointer; transition: background 0.15s; }
input[type="range"]::-webkit-slider-thumb { -webkit-appearance: none; width: 16px; height: 16px; border-radius: 50%; background: var(--color-void); cursor: pointer; transition: transform 0.1s; }
input[type="range"]::-webkit-slider-thumb:hover { transform: scale(1.2); }
input[type="range"]::-moz-range-thumb { width: 16px; height: 16px; border-radius: 50%; background: var(--color-void); border: none; cursor: pointer; }
.control-value { font-size: 13px; font-weight: 600; color: var(--color-void); min-width: 80px; text-align: right; font-variant-numeric: tabular-nums; }

/* READOUT */
.readout { margin-top: var(--spacing-16); padding: var(--spacing-16) var(--spacing-20); background: var(--color-void); border-radius: var(--radius-cards); display: flex; gap: var(--spacing-32); flex-wrap: wrap; }
.readout-item { display: flex; flex-direction: column; gap: 2px; }
.readout-key { font-size: 11px; font-weight: 500; color: rgba(255,255,255,0.5); letter-spacing: 0.05em; text-transform: uppercase; }
.readout-val { font-size: 18px; font-weight: 600; color: #fff; font-variant-numeric: tabular-nums; letter-spacing: -0.01em; }

/* HIGHLIGHT */
.highlight { border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); padding: var(--spacing-24); margin-top: var(--spacing-24); }
.highlight-title { font-size: 14px; font-weight: 600; color: var(--color-void); margin-bottom: var(--spacing-8); }
.highlight-body { font-size: 14px; color: var(--color-graphite); line-height: 1.6; }

/* LEGEND */
.legend { display: flex; gap: var(--spacing-20); margin-top: var(--spacing-12); flex-wrap: wrap; }
.legend-item { display: flex; align-items: center; gap: var(--spacing-8); font-size: 13px; color: var(--color-graphite); }
.legend-dot { width: 12px; height: 3px; border-radius: 2px; flex-shrink: 0; }

/* TAKEAWAYS */
.takeaways { padding: var(--spacing-80) 0; }
.takeaway-grid { display: grid; grid-template-columns: repeat(3,1fr); gap: var(--spacing-32); margin-top: var(--spacing-40); }
.takeaway-card { border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); padding: var(--spacing-32); }
.takeaway-number { font-size: 40px; font-weight: 600; color: var(--color-fog-border); line-height: 1; margin-bottom: var(--spacing-16); letter-spacing: -0.03em; }
.takeaway-title { font-size: 16px; font-weight: 600; color: var(--color-void); margin-bottom: var(--spacing-8); line-height: 1.3; }
.takeaway-body { font-size: 14px; color: var(--color-graphite); line-height: 1.6; }

/* FOOTER */
footer { border-top: 1px solid var(--color-fog-border); padding: var(--spacing-40) 0; }
.footer-inner { max-width: 1200px; margin: 0 auto; padding: 0 var(--spacing-40); display: flex; align-items: center; justify-content: space-between; }
.footer-text { font-size: 13px; color: var(--color-graphite); }

/* CHALLENGE */
.challenge-btn {
  display: inline-flex; align-items: center; gap: 6px;
  background: #000; color: #fff;
  font-size: 12px; font-weight: 600; font-family: var(--font-inter);
  border: none; border-radius: 9999px;
  padding: 6px 14px; cursor: pointer;
  transition: opacity 0.15s, transform 0.1s;
  letter-spacing: 0.02em;
}
.challenge-btn:hover { opacity: 0.8; transform: scale(1.03); }
.challenge-panel { display: none; margin-top: 12px; background: #f1f1f1; border-radius: var(--radius-cards); padding: 16px 20px; }
.challenge-panel.open { display: block; }
.challenge-goal { font-size: 13px; font-weight: 600; color: #000; margin-bottom: 6px; }
.challenge-desc { font-size: 13px; color: var(--color-graphite); line-height: 1.55; }
.challenge-status { margin-top: 10px; font-size: 13px; font-weight: 600; color: var(--color-graphite); }
.challenge-status.success { color: #000; }

/* TOAST */
.toast {
  position: fixed; bottom: 32px; left: 50%; transform: translateX(-50%) translateY(20px);
  background: #000; color: #fff;
  font-size: 14px; font-weight: 500; font-family: var(--font-inter);
  padding: 12px 24px; border-radius: 9999px;
  opacity: 0; pointer-events: none;
  transition: opacity 0.3s, transform 0.3s;
  z-index: 999; max-width: 480px; text-align: center;
}
.toast.show { opacity: 1; transform: translateX(-50%) translateY(0); }

/* GUIDED EXPLORATION */
.explore-toggle {
  display: inline-flex; align-items: center; gap: 6px;
  background: none; border: 1px solid var(--color-fog-border);
  border-radius: 9999px; padding: 6px 14px;
  font-size: 12px; font-weight: 500; font-family: var(--font-inter);
  cursor: pointer; color: var(--color-graphite); margin-top: 12px;
  transition: border-color 0.15s, color 0.15s;
}
.explore-toggle:hover { border-color: #000; color: #000; }
.explore-panel { display: none; margin-top: 12px; border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); padding: 16px 20px; }
.explore-panel.open { display: block; }
.explore-steps { list-style: none; counter-reset: step; }
.explore-steps li { counter-increment: step; display: flex; gap: 12px; padding: 8px 0; border-bottom: 1px solid var(--color-fog-border); font-size: 13px; color: var(--color-graphite); line-height: 1.55; }
.explore-steps li:last-child { border-bottom: none; }
.explore-steps li::before { content: counter(step); font-size: 11px; font-weight: 700; color: #fff; background: #000; border-radius: 50%; min-width: 20px; height: 20px; display: flex; align-items: center; justify-content: center; flex-shrink: 0; margin-top: 1px; }
.explore-insight { margin-top: 10px; padding: 10px 14px; background: #000; color: #fff; border-radius: var(--radius-cards); font-size: 13px; line-height: 1.5; }
.explore-insight strong { font-weight: 600; }

/* WHAT-IF CARDS */
.whatif-grid { display: flex; gap: 12px; margin-top: 16px; flex-wrap: wrap; }
.whatif-card { flex: 1; min-width: 200px; background: #f1f1f1; border-radius: var(--radius-cards); padding: 14px 16px; cursor: pointer; transition: background 0.15s; }
.whatif-card:hover { background: #e8e8e8; }
.whatif-q { font-size: 13px; font-weight: 600; color: #000; margin-bottom: 4px; }
.whatif-hint { font-size: 12px; color: var(--color-graphite); line-height: 1.5; display: none; margin-top: 6px; }
.whatif-card.revealed .whatif-hint { display: block; }
.whatif-arrow { font-size: 11px; color: var(--color-ash); margin-top: 4px; }
.whatif-card.revealed .whatif-arrow { display: none; }

/* QUIZ */
.quiz-block { margin-top: 20px; border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); overflow: hidden; }
.quiz-q { padding: 14px 18px; font-size: 14px; font-weight: 600; color: #000; background: #fafafa; border-bottom: 1px solid var(--color-fog-border); }
.quiz-options { padding: 12px 18px; display: flex; flex-direction: column; gap: 8px; }
.quiz-opt {
  font-size: 13px; padding: 8px 12px; border-radius: var(--radius-cards);
  border: 1px solid var(--color-fog-border); cursor: pointer;
  background: #fff; transition: background 0.12s, border-color 0.12s;
  font-family: var(--font-inter); text-align: left;
}
.quiz-opt:hover { background: #f1f1f1; }
.quiz-opt.correct { background: #000; color: #fff; border-color: #000; }
.quiz-opt.wrong { background: #f1f1f1; color: var(--color-ash); text-decoration: line-through; }
.quiz-explain { padding: 10px 18px 14px; font-size: 13px; color: var(--color-graphite); display: none; line-height: 1.55; }
.quiz-explain.show { display: block; }

/* DERIVATION TOGGLE */
details.derivation { margin: var(--spacing-16) 0; border: 1px solid var(--color-fog-border); border-radius: var(--radius-cards); overflow: hidden; }
details.derivation summary { padding: 10px 16px; font-size: 13px; font-weight: 500; color: var(--color-graphite); cursor: pointer; list-style: none; display: flex; align-items: center; gap: 6px; background: #fafafa; transition: background 0.15s; }
details.derivation summary::-webkit-details-marker { display: none; }
details.derivation summary::before { content: '▶'; font-size: 10px; transition: transform 0.2s; }
details.derivation[open] summary::before { transform: rotate(90deg); }
details.derivation summary:hover { background: #f1f1f1; }
.deriv-body { padding: 16px 20px; font-size: 13px; color: var(--color-graphite); line-height: 1.7; }
.deriv-step { padding: 8px 0; border-bottom: 1px solid var(--color-fog-border); display: flex; flex-direction: column; gap: 4px; }
.deriv-step:last-child { border-bottom: none; }
.deriv-step-num { font-size: 11px; font-weight: 700; color: #fff; background: #000; border-radius: 50%; min-width: 20px; height: 20px; display: inline-flex; align-items: center; justify-content: center; flex-shrink: 0; }
.deriv-step-row { display: flex; gap: 10px; align-items: flex-start; }
.deriv-step-text { flex: 1; }
.deriv-meaning { font-size: 12px; color: var(--color-ash); font-style: italic; margin-top: 2px; }

/* TEXT WRAP */
p, .section-body p, .concept-list li, .highlight-body, .takeaway-body,
.deriv-step-text, .deriv-meaning, .explore-insight, .whatif-hint,
.challenge-desc, .quiz-explain {
  word-break: keep-all;
  overflow-wrap: break-word;
  line-height: 1.7;
}

/* RESPONSIVE */
@media (max-width: 768px) {
  nav { padding: 0 var(--spacing-20); }
  .page { padding: 0 var(--spacing-20); }
  .two-col { grid-template-columns: 1fr; gap: var(--spacing-40); }
  .hero { padding: var(--spacing-64) 0; }
  .hero-title { font-size: 36px; letter-spacing: -0.8px; }
  .takeaway-grid { grid-template-columns: 1fr; }
  .control-row { flex-wrap: wrap; }
  .control-label { min-width: 100%; }
  .footer-inner { flex-direction: column; gap: var(--spacing-8); text-align: center; }
  nav .nav-links { display: none; }
}