/* ================================================
   AAYUSH'S LEARNING SPACE — Design System
   Clean, minimal, premium light theme
   ================================================ */

@import url('https://fonts.googleapis.com/css2?family=Instrument+Serif:ital@0;1&family=Inter:wght@300;400;500;600&family=JetBrains+Mono:wght@400;500&display=swap');

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --ink:       #0d0d0d;
  --ink-2:     #3a3a3a;
  --ink-3:     #6b6b6b;
  --ink-4:     #9a9a9a;
  --paper:     #fafaf8;
  --paper-2:   #f3f2ef;
  --paper-3:   #eceae5;
  --border:    #e2e0da;
  --border-2:  #d0cec8;
  --accent:    #1a1a2e;
  --accent-2:  #16213e;
  --gold:      #c9a84c;
  --gold-light:#f0d98a;
  --green:     #2d6a4f;
  --green-bg:  #d8f3dc;
  --red:       #9b2226;
  --red-bg:    #fde8e8;
  --blue:      #1d3557;
  --blue-bg:   #e8f0fe;
  --radius:    14px;
  --radius-sm: 8px;
  --shadow-sm: 0 1px 4px rgba(0,0,0,0.06), 0 2px 12px rgba(0,0,0,0.04);
  --shadow:    0 2px 8px rgba(0,0,0,0.07), 0 8px 32px rgba(0,0,0,0.06);
  --shadow-lg: 0 4px 16px rgba(0,0,0,0.08), 0 16px 48px rgba(0,0,0,0.08);
  --transition: 0.2s cubic-bezier(0.4,0,0.2,1);
}

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

body {
  font-family: 'Inter', system-ui, sans-serif;
  background: var(--paper);
  color: var(--ink);
  line-height: 1.7;
  -webkit-font-smoothing: antialiased;
}

/* ── TYPOGRAPHY ── */
.serif { font-family: 'Instrument Serif', Georgia, serif; }
.mono  { font-family: 'JetBrains Mono', monospace; }

h1,h2,h3,h4 { line-height: 1.25; font-weight: 600; color: var(--ink); }
h1 { font-size: clamp(2rem, 5vw, 3.5rem); }
h2 { font-size: clamp(1.4rem, 3vw, 2rem); }
h3 { font-size: 1.15rem; }
p  { color: var(--ink-2); }

a { color: inherit; text-decoration: none; }

/* ── LAYOUT ── */
.container { max-width: 860px; margin: 0 auto; padding: 0 28px; }
.container-wide { max-width: 1100px; margin: 0 auto; padding: 0 28px; }

/* ── NAV ── */
.nav {
  position: sticky; top: 0; z-index: 100;
  background: rgba(250,250,248,0.92);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border);
  padding: 0 28px;
}
.nav-inner {
  max-width: 1100px; margin: 0 auto;
  display: flex; align-items: center; justify-content: space-between;
  height: 56px;
}
.nav-brand {
  display: flex; align-items: center; gap: 10px;
  font-size: 14px; font-weight: 600; color: var(--ink);
  text-decoration: none;
}
.nav-brand-dot {
  width: 28px; height: 28px; border-radius: 8px;
  background: var(--ink); color: var(--paper);
  display: flex; align-items: center; justify-content: center;
  font-size: 14px; font-weight: 700;
  font-family: 'Instrument Serif', serif;
}
.nav-right { display: flex; align-items: center; gap: 20px; }
.nav-score {
  display: flex; align-items: center; gap: 6px;
  font-size: 13px; color: var(--ink-3);
}
.nav-score-val {
  font-family: 'JetBrains Mono', monospace;
  font-size: 14px; font-weight: 500; color: var(--ink);
}
.nav-back {
  font-size: 13px; color: var(--ink-3);
  display: flex; align-items: center; gap: 4px;
  transition: color var(--transition);
}
.nav-back:hover { color: var(--ink); }
.progress-bar {
  height: 2px; background: var(--border);
  position: absolute; bottom: 0; left: 0; right: 0;
}
.progress-fill {
  height: 100%; background: var(--ink);
  width: 0%; transition: width 0.5s ease;
}

/* ── HERO (INDEX) ── */
.hero {
  padding: 80px 28px 64px;
  max-width: 1100px; margin: 0 auto;
}
.hero-eyebrow {
  display: inline-flex; align-items: center; gap: 8px;
  font-size: 12px; font-weight: 500; letter-spacing: 0.08em;
  text-transform: uppercase; color: var(--ink-3);
  margin-bottom: 24px;
}
.hero-eyebrow-line {
  width: 24px; height: 1px; background: var(--ink-3);
}
.hero-title {
  font-family: 'Instrument Serif', serif;
  font-size: clamp(2.8rem, 6vw, 5rem);
  font-weight: 400; line-height: 1.1;
  color: var(--ink); margin-bottom: 16px;
  letter-spacing: -0.02em;
}
.hero-title em { font-style: italic; color: var(--ink-3); }
.hero-sub {
  font-size: 17px; color: var(--ink-3);
  max-width: 480px; line-height: 1.7;
  margin-bottom: 48px;
}
.hero-meta {
  display: flex; gap: 32px; flex-wrap: wrap;
}
.hero-meta-item {
  display: flex; flex-direction: column; gap: 2px;
}
.hero-meta-num {
  font-family: 'Instrument Serif', serif;
  font-size: 2rem; color: var(--ink); line-height: 1;
}
.hero-meta-label {
  font-size: 12px; color: var(--ink-4);
  text-transform: uppercase; letter-spacing: 0.06em;
}

/* ── CHAPTER GRID ── */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 16px;
  padding: 0 28px 80px;
  max-width: 1100px; margin: 0 auto;
}
.chapter-card {
  background: white;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 28px;
  cursor: pointer;
  transition: all var(--transition);
  text-decoration: none;
  display: block;
  position: relative;
  overflow: hidden;
}
.chapter-card::after {
  content: '';
  position: absolute; inset: 0;
  background: var(--ink);
  opacity: 0;
  transition: opacity var(--transition);
  border-radius: var(--radius);
}
.chapter-card:hover { transform: translateY(-3px); box-shadow: var(--shadow-lg); border-color: var(--border-2); }
.chapter-card.completed { border-color: var(--green); }
.chapter-card.completed .card-num { color: var(--green); }
.card-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px; font-weight: 500;
  color: var(--ink-4); letter-spacing: 0.06em;
  margin-bottom: 16px; display: block;
}
.card-icon { font-size: 28px; margin-bottom: 14px; display: block; }
.card-title {
  font-size: 16px; font-weight: 600;
  color: var(--ink); margin-bottom: 8px; line-height: 1.3;
}
.card-desc {
  font-size: 13px; color: var(--ink-3);
  line-height: 1.6; margin-bottom: 20px;
}
.card-footer {
  display: flex; align-items: center; justify-content: space-between;
}
.card-qs {
  font-size: 12px; color: var(--ink-4);
  font-family: 'JetBrains Mono', monospace;
}
.card-arrow {
  width: 28px; height: 28px; border-radius: 50%;
  border: 1px solid var(--border);
  display: flex; align-items: center; justify-content: center;
  font-size: 13px; color: var(--ink-3);
  transition: all var(--transition);
}
.chapter-card:hover .card-arrow {
  background: var(--ink); border-color: var(--ink); color: white;
}
.card-progress-bar {
  height: 3px; background: var(--paper-3);
  border-radius: 2px; margin-top: 16px; overflow: hidden;
}
.card-progress-fill {
  height: 100%; background: var(--green);
  border-radius: 2px; transition: width 0.4s ease;
}

/* ── TOPIC PAGE LAYOUT ── */
.topic-hero {
  padding: 56px 28px 48px;
  border-bottom: 1px solid var(--border);
  background: white;
}
.topic-hero-inner { max-width: 860px; margin: 0 auto; }
.topic-chapter-label {
  font-size: 12px; font-weight: 500; letter-spacing: 0.08em;
  text-transform: uppercase; color: var(--ink-4);
  margin-bottom: 12px; display: flex; align-items: center; gap: 8px;
}
.topic-icon-large { font-size: 40px; margin-bottom: 16px; display: block; }
.topic-title {
  font-family: 'Instrument Serif', serif;
  font-size: clamp(1.8rem, 4vw, 3rem);
  font-weight: 400; color: var(--ink);
  margin-bottom: 12px; letter-spacing: -0.01em;
}
.topic-intro {
  font-size: 16px; color: var(--ink-3);
  max-width: 600px; line-height: 1.75;
}
.topic-nav-pills {
  display: flex; gap: 8px; flex-wrap: wrap;
  padding: 16px 28px; border-bottom: 1px solid var(--border);
  background: var(--paper-2); max-width: 100%; overflow-x: auto;
}
.pill {
  font-size: 12px; font-weight: 500;
  padding: 6px 14px; border-radius: 100px;
  border: 1px solid var(--border);
  background: white; color: var(--ink-3);
  cursor: pointer; white-space: nowrap;
  transition: all var(--transition);
  text-decoration: none; display: inline-block;
}
.pill:hover, .pill.active {
  background: var(--ink); color: white; border-color: var(--ink);
}

/* ── CONTENT SECTIONS ── */
.content-body { padding: 48px 28px 80px; }
.content-section { max-width: 860px; margin: 0 auto 56px; }
.section-label {
  font-size: 11px; font-weight: 600; letter-spacing: 0.1em;
  text-transform: uppercase; color: var(--ink-4);
  margin-bottom: 20px; display: flex; align-items: center; gap: 10px;
}
.section-label::after {
  content: ''; flex: 1; height: 1px; background: var(--border);
}
.section-title {
  font-family: 'Instrument Serif', serif;
  font-size: 1.6rem; font-weight: 400;
  color: var(--ink); margin-bottom: 16px;
}

/* ── PROSE ── */
.prose p {
  font-size: 15px; color: var(--ink-2);
  line-height: 1.85; margin-bottom: 16px;
}
.prose strong { color: var(--ink); font-weight: 600; }
.prose em { color: var(--ink-3); }

/* ── CONCEPT CARDS ── */
.concept-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
  gap: 12px; margin: 24px 0;
}
.concept-card {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 20px;
  transition: box-shadow var(--transition);
}
.concept-card:hover { box-shadow: var(--shadow); }
.concept-card-icon { font-size: 24px; margin-bottom: 10px; }
.concept-card-title { font-size: 14px; font-weight: 600; margin-bottom: 6px; }
.concept-card-text { font-size: 13px; color: var(--ink-3); line-height: 1.6; }
.concept-card-tag {
  display: inline-block; margin-top: 10px;
  font-size: 11px; font-weight: 500; letter-spacing: 0.04em;
  color: var(--ink-4); background: var(--paper-2);
  padding: 2px 8px; border-radius: 4px;
}

/* ── INFO BLOCKS ── */
.info-block {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 24px 28px;
  margin: 20px 0;
}
.info-block h3 { font-size: 15px; font-weight: 600; margin-bottom: 14px; }
.info-block p { font-size: 14px; color: var(--ink-2); line-height: 1.75; }

.two-col { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; margin: 20px 0; }

/* ── CALLOUT ── */
.callout {
  display: flex; gap: 14px; align-items: flex-start;
  padding: 18px 22px; border-radius: var(--radius);
  margin: 20px 0; border-left: 3px solid;
}
.callout-note  { background: var(--blue-bg);  border-color: #457b9d; }
.callout-tip   { background: var(--green-bg); border-color: var(--green); }
.callout-warn  { background: #fff8e1;          border-color: #f4a261; }
.callout-fact  { background: var(--paper-2);   border-color: var(--border-2); }
.callout-icon  { font-size: 18px; flex-shrink: 0; margin-top: 1px; }
.callout p     { font-size: 14px; line-height: 1.7; color: var(--ink-2); }
.callout strong { color: var(--ink); }

/* ── SPECTRUM ── */
.spectrum-wrap { margin: 24px 0; }
.spectrum-bar {
  display: flex; border-radius: 10px;
  overflow: hidden; height: 48px; margin-bottom: 10px;
}
.spec-seg {
  flex: 1; display: flex; align-items: center; justify-content: center;
  font-size: 10px; font-weight: 600; color: rgba(255,255,255,0.9);
  letter-spacing: 0.03em;
}
.seg-gamma  { background: #1a0533; }
.seg-xray   { background: #2d1b69; }
.seg-uv     { background: #5b21b6; }
.seg-vis    { background: linear-gradient(90deg,#7c3aed,#2563eb,#16a34a,#ca8a04,#ea580c,#dc2626); }
.seg-ir     { background: #b45309; }
.seg-micro  { background: #92400e; }
.seg-radio  { background: #78350f; }
.spectrum-labels {
  display: flex; justify-content: space-between;
  font-size: 11px; color: var(--ink-4);
}

/* ── LAYERS TABLE ── */
.layers-table { width: 100%; border-collapse: collapse; margin: 20px 0; }
.layers-table th {
  text-align: left; font-size: 11px; font-weight: 600;
  letter-spacing: 0.06em; text-transform: uppercase;
  color: var(--ink-4); padding: 10px 16px;
  border-bottom: 2px solid var(--border);
  background: var(--paper-2);
}
.layers-table td {
  padding: 14px 16px; font-size: 13px; color: var(--ink-2);
  border-bottom: 1px solid var(--border); vertical-align: top;
}
.layers-table tr:last-child td { border-bottom: none; }
.layers-table tr:hover td { background: var(--paper-2); }
.layer-name { font-weight: 600; color: var(--ink); }
.layer-alt  { font-family: 'JetBrains Mono', monospace; font-size: 12px; color: var(--ink-3); }

/* ── FORMULA CARD ── */
.formula-card {
  background: var(--ink); color: white;
  border-radius: var(--radius); padding: 36px;
  text-align: center; margin: 24px 0;
}
.formula-label {
  font-size: 11px; font-weight: 500; letter-spacing: 0.1em;
  text-transform: uppercase; color: rgba(255,255,255,0.5);
  margin-bottom: 16px;
}
.formula-eq {
  font-family: 'Instrument Serif', serif;
  font-size: 3rem; color: white; margin-bottom: 24px;
  letter-spacing: 0.05em;
}
.formula-vars {
  display: flex; justify-content: center; gap: 28px; flex-wrap: wrap;
}
.formula-var { font-size: 13px; color: rgba(255,255,255,0.6); }
.formula-var span {
  font-family: 'JetBrains Mono', monospace;
  color: var(--gold-light); font-weight: 500;
}
.formula-note {
  margin-top: 16px; font-size: 12px;
  color: rgba(255,255,255,0.4); letter-spacing: 0.02em;
}

/* ── WORKED EXAMPLE ── */
.worked-example {
  border: 1px solid var(--border); border-radius: var(--radius);
  overflow: hidden; margin: 16px 0;
}
.we-q {
  background: var(--paper-2); padding: 18px 22px;
  border-bottom: 1px solid var(--border);
  display: flex; gap: 14px; align-items: flex-start;
}
.we-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px; font-weight: 500; color: var(--ink-4);
  background: white; border: 1px solid var(--border);
  padding: 3px 8px; border-radius: 4px; flex-shrink: 0; margin-top: 2px;
}
.we-q p { font-size: 14px; color: var(--ink-2); line-height: 1.6; }
.we-q strong { color: var(--ink); }
.we-steps { padding: 18px 22px; display: flex; flex-direction: column; gap: 8px; }
.we-step {
  display: flex; gap: 12px; align-items: flex-start;
  font-size: 13px; color: var(--ink-2); padding: 10px 14px;
  background: var(--paper-2); border-radius: var(--radius-sm);
}
.we-step-n {
  width: 20px; height: 20px; border-radius: 50%;
  background: var(--border); color: var(--ink-3);
  display: flex; align-items: center; justify-content: center;
  font-size: 11px; font-weight: 600; flex-shrink: 0;
}
.we-step.answer {
  background: #f0fdf4; border: 1px solid #bbf7d0;
}
.we-step.answer .we-step-n {
  background: var(--green); color: white; border: none;
}
.we-step.answer p { color: var(--ink); font-weight: 600; }

/* ── CALCULATOR ── */
.calc-card {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 28px; margin: 24px 0;
}
.calc-card h3 { font-size: 15px; font-weight: 600; margin-bottom: 6px; }
.calc-card > p { font-size: 13px; color: var(--ink-3); margin-bottom: 20px; }
.calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin-bottom: 16px; }
.calc-field label {
  display: block; font-size: 11px; font-weight: 600;
  text-transform: uppercase; letter-spacing: 0.06em;
  color: var(--ink-4); margin-bottom: 6px;
}
.calc-field input {
  width: 100%; padding: 10px 14px;
  border: 1px solid var(--border); border-radius: var(--radius-sm);
  font-size: 14px; font-family: 'JetBrains Mono', monospace;
  color: var(--ink); background: var(--paper);
  transition: border-color var(--transition);
}
.calc-field input:focus { outline: none; border-color: var(--ink); }
.btn-calc {
  background: var(--ink); color: white;
  border: none; padding: 11px 24px;
  border-radius: var(--radius-sm); font-size: 13px;
  font-weight: 600; cursor: pointer;
  transition: opacity var(--transition);
}
.btn-calc:hover { opacity: 0.8; }
.calc-result {
  margin-top: 14px; padding: 12px 16px;
  border-radius: var(--radius-sm); font-size: 14px;
  font-family: 'JetBrains Mono', monospace;
  display: none;
}
.calc-result.ok  { background: var(--green-bg); color: var(--green); display: block; }
.calc-result.err { background: var(--red-bg);   color: var(--red);   display: block; }

/* ── ALBEDO TABLE ── */
.data-table { width: 100%; border-collapse: collapse; margin: 16px 0; }
.data-table th {
  text-align: left; font-size: 11px; font-weight: 600;
  letter-spacing: 0.06em; text-transform: uppercase;
  color: var(--ink-4); padding: 10px 14px;
  border-bottom: 2px solid var(--border); background: var(--paper-2);
}
.data-table td {
  padding: 12px 14px; font-size: 13px; color: var(--ink-2);
  border-bottom: 1px solid var(--border);
}
.data-table tr:last-child td { border-bottom: none; }
.data-table tr:hover td { background: var(--paper-2); }
.val-high { color: #1d4ed8; font-weight: 600; font-family: 'JetBrains Mono', monospace; }
.val-mid  { color: #b45309; font-weight: 600; font-family: 'JetBrains Mono', monospace; }
.val-low  { color: var(--red); font-weight: 600; font-family: 'JetBrains Mono', monospace; }

/* ── PROCESS FLOW ── */
.process-flow { display: flex; flex-direction: column; gap: 4px; margin: 16px 0; }
.pf-item {
  display: flex; align-items: flex-start; gap: 12px;
  padding: 12px 16px; background: var(--paper-2);
  border-radius: var(--radius-sm); font-size: 13px; color: var(--ink-2);
}
.pf-item strong { color: var(--ink); }
.pf-arrow { text-align: center; color: var(--ink-4); font-size: 14px; padding: 2px 0; }
.pf-num {
  width: 22px; height: 22px; border-radius: 50%;
  background: var(--ink); color: white;
  display: flex; align-items: center; justify-content: center;
  font-size: 11px; font-weight: 700; flex-shrink: 0;
}

/* ── CYCLE STEPS ── */
.cycle-row {
  display: grid; grid-template-columns: repeat(4, 1fr);
  gap: 12px; margin: 20px 0;
}
.cycle-step {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 18px 14px;
  text-align: center; position: relative;
}
.cycle-step-icon { font-size: 28px; margin-bottom: 8px; }
.cycle-step-label { font-size: 13px; font-weight: 600; color: var(--ink); margin-bottom: 6px; }
.cycle-step-text { font-size: 12px; color: var(--ink-3); line-height: 1.5; }
.cycle-connector {
  position: absolute; right: -8px; top: 50%;
  transform: translateY(-50%);
  font-size: 14px; color: var(--ink-4); z-index: 1;
}

/* ── KEELING CHART ── */
.keeling-wrap {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 24px; margin: 20px 0;
}
.keeling-wrap h3 { font-size: 15px; font-weight: 600; margin-bottom: 6px; }
.keeling-wrap > p { font-size: 13px; color: var(--ink-3); margin-bottom: 20px; }
.keeling-chart {
  display: flex; align-items: flex-end; gap: 6px;
  height: 120px; padding-bottom: 24px; position: relative;
}
.kc-bar {
  flex: 1; border-radius: 4px 4px 0 0;
  background: linear-gradient(180deg, #6366f1, #4338ca);
  height: var(--h); position: relative; transition: filter 0.2s;
}
.kc-bar:hover { filter: brightness(1.15); }
.kc-bar span {
  position: absolute; bottom: -20px; left: 50%;
  transform: translateX(-50%);
  font-size: 10px; color: var(--ink-4); white-space: nowrap;
  font-family: 'JetBrains Mono', monospace;
}
.kc-bar.current { background: linear-gradient(180deg, #f59e0b, #d97706); }
.kc-note { font-size: 12px; color: var(--ink-4); margin-top: 8px; }

/* ── PRESSURE BELTS ── */
.pressure-belts { display: flex; flex-direction: column; gap: 4px; margin: 16px 0; }
.pb-item {
  padding: 10px 16px; border-radius: var(--radius-sm);
  font-size: 13px; font-weight: 500; text-align: center;
  border: 1px solid;
}
.pb-high   { background: #eff6ff; border-color: #bfdbfe; color: #1d4ed8; }
.pb-low    { background: #fef2f2; border-color: #fecaca; color: #991b1b; }
.pb-eq     { background: #fff7ed; border-color: #fed7aa; color: #9a3412; }

/* ── HABER BOX ── */
.haber-box {
  background: var(--paper-2); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 20px; margin: 16px 0;
}
.haber-conditions { display: flex; gap: 12px; flex-wrap: wrap; margin-bottom: 14px; }
.haber-cond {
  font-size: 12px; color: var(--ink-3);
  background: white; border: 1px solid var(--border);
  padding: 5px 12px; border-radius: 100px;
}
.haber-eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 16px; font-weight: 500; color: var(--ink);
  text-align: center; padding: 12px;
  background: white; border-radius: var(--radius-sm);
  border: 1px solid var(--border);
}

/* ── AGREEMENTS TABLE ── */
.ag-table { width: 100%; border-collapse: collapse; margin: 16px 0; }
.ag-table th {
  text-align: left; font-size: 11px; font-weight: 600;
  letter-spacing: 0.06em; text-transform: uppercase;
  color: var(--ink-4); padding: 10px 14px;
  border-bottom: 2px solid var(--border); background: var(--paper-2);
}
.ag-table td {
  padding: 14px; font-size: 13px; color: var(--ink-2);
  border-bottom: 1px solid var(--border); vertical-align: top;
}
.ag-table tr:last-child td { border-bottom: none; }
.ag-table tr:hover td { background: var(--paper-2); }
.ag-name { font-weight: 600; color: var(--ink); }

/* ── QUIZ SECTION ── */
.quiz-section {
  background: var(--paper-2); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 32px; margin-top: 48px;
}
.quiz-header { margin-bottom: 28px; }
.quiz-eyebrow {
  font-size: 11px; font-weight: 600; letter-spacing: 0.1em;
  text-transform: uppercase; color: var(--ink-4);
  margin-bottom: 8px; display: block;
}
.quiz-header h2 {
  font-family: 'Instrument Serif', serif;
  font-size: 1.6rem; font-weight: 400; color: var(--ink);
}
.quiz-score-row {
  display: flex; align-items: center; gap: 12px;
  margin-bottom: 24px; padding: 14px 18px;
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius-sm);
}
.qs-label { font-size: 13px; color: var(--ink-3); }
.qs-val {
  font-family: 'JetBrains Mono', monospace;
  font-size: 18px; font-weight: 500; color: var(--ink);
}
.qs-sep { color: var(--border-2); }
.qs-total { font-family: 'JetBrains Mono', monospace; font-size: 14px; color: var(--ink-4); }
.qs-pct {
  margin-left: auto; font-size: 12px; color: var(--ink-4);
  font-family: 'JetBrains Mono', monospace;
}

/* ── QUESTION CARD ── */
.q-card {
  background: white; border: 1px solid var(--border);
  border-radius: var(--radius); padding: 24px;
  margin-bottom: 14px; transition: box-shadow var(--transition);
}
.q-card:hover { box-shadow: var(--shadow-sm); }
.q-meta {
  font-size: 11px; font-weight: 500; letter-spacing: 0.06em;
  text-transform: uppercase; color: var(--ink-4); margin-bottom: 10px;
}
.q-text { font-size: 15px; font-weight: 500; color: var(--ink); margin-bottom: 18px; line-height: 1.6; }
.q-options { display: flex; flex-direction: column; gap: 8px; }
.q-opt {
  display: flex; align-items: center; gap: 12px;
  padding: 12px 16px; border: 1px solid var(--border);
  border-radius: var(--radius-sm); cursor: pointer;
  font-size: 14px; color: var(--ink-2);
  background: var(--paper); transition: all var(--transition);
  text-align: left; font-family: 'Inter', sans-serif; width: 100%;
}
.q-opt:hover:not(:disabled) { border-color: var(--ink); background: white; color: var(--ink); }
.q-opt-letter {
  width: 24px; height: 24px; border-radius: 6px;
  border: 1px solid var(--border); background: white;
  display: flex; align-items: center; justify-content: center;
  font-size: 11px; font-weight: 600; color: var(--ink-4);
  flex-shrink: 0; transition: all var(--transition);
}
.q-opt.correct { border-color: #86efac; background: #f0fdf4; color: var(--ink); }
.q-opt.correct .q-opt-letter { background: var(--green); border-color: var(--green); color: white; }
.q-opt.wrong   { border-color: #fca5a5; background: #fef2f2; color: var(--ink); }
.q-opt.wrong   .q-opt-letter { background: var(--red);   border-color: var(--red);   color: white; }
.q-opt:disabled { cursor: default; }

.q-feedback {
  margin-top: 14px; padding: 14px 18px;
  border-radius: var(--radius-sm); font-size: 13px;
  line-height: 1.75; display: none;
}
.q-feedback.show { display: block; }
.q-feedback.ok  { background: #f0fdf4; border: 1px solid #bbf7d0; color: #166534; }
.q-feedback.bad { background: #fef2f2; border: 1px solid #fecaca; color: #991b1b; }

/* Numerical */
.q-num-row { display: flex; gap: 10px; align-items: center; margin-bottom: 8px; }
.q-num-input {
  flex: 1; padding: 10px 14px;
  border: 1px solid var(--border); border-radius: var(--radius-sm);
  font-size: 14px; font-family: 'JetBrains Mono', monospace;
  color: var(--ink); background: var(--paper);
  transition: border-color var(--transition);
}
.q-num-input:focus { outline: none; border-color: var(--ink); }
.q-num-unit { font-size: 13px; color: var(--ink-3); white-space: nowrap; }
.btn-check {
  background: var(--ink); color: white;
  border: none; padding: 10px 20px;
  border-radius: var(--radius-sm); font-size: 13px;
  font-weight: 600; cursor: pointer; white-space: nowrap;
  transition: opacity var(--transition);
}
.btn-check:hover { opacity: 0.8; }
.q-hint {
  font-size: 12px; color: var(--ink-4);
  padding: 8px 12px; background: var(--paper-2);
  border-radius: var(--radius-sm); margin-bottom: 12px;
}

/* ── TOPIC FOOTER NAV ── */
.topic-footer-nav {
  display: flex; justify-content: space-between; align-items: center;
  padding: 28px; border-top: 1px solid var(--border);
  background: white; margin-top: 0;
}
.tfn-btn {
  display: flex; align-items: center; gap: 8px;
  font-size: 14px; color: var(--ink-3);
  text-decoration: none; transition: color var(--transition);
  padding: 10px 18px; border: 1px solid var(--border);
  border-radius: var(--radius-sm); background: white;
}
.tfn-btn:hover { color: var(--ink); border-color: var(--ink); }
.tfn-btn.primary {
  background: var(--ink); color: white; border-color: var(--ink);
}
.tfn-btn.primary:hover { opacity: 0.85; }

/* ── FOOTER ── */
.footer {
  background: var(--paper-2); border-top: 1px solid var(--border);
  padding: 32px 28px; text-align: center;
}
.footer p { font-size: 13px; color: var(--ink-4); }

/* ── RESPONSIVE ── */
@media (max-width: 700px) {
  .two-col { grid-template-columns: 1fr; }
  .cycle-row { grid-template-columns: 1fr 1fr; }
  .calc-grid { grid-template-columns: 1fr; }
  .formula-eq { font-size: 2.2rem; }
  .formula-vars { flex-direction: column; align-items: center; gap: 8px; }
  .chapter-grid { grid-template-columns: 1fr; }
  .concept-grid { grid-template-columns: 1fr 1fr; }
  .hero-meta { gap: 20px; }
  .layers-table { font-size: 12px; }
  .layers-table th, .layers-table td { padding: 8px 10px; }
}