/* ── Design tokens ──────────────────────────────────────────────────── */
:root {
  --bg:         #0d1117;
  --bg2:        #161b22;
  --bg3:        #1c2430;
  --bg4:        #21262d;
  --border:     #30363d;
  --border2:    #3d4451;
  --text:       #e6edf3;
  --text2:      #8b949e;
  --text3:      #484f58;
  --amber:      #f0a500;
  --amber2:     #ffc53d;
  --amber-dim:  rgba(240,165,0,0.12);
  --teal:       #39c5cf;
  --teal-dim:   rgba(57,197,207,0.1);
  --red:        #f85149;
  --red-dim:    rgba(248,81,73,0.1);
  --green:      #3fb950;
  --green-dim:  rgba(63,185,80,0.1);
  --blue:       #58a6ff;
  --blue-dim:   rgba(88,166,255,0.1);
  --radius:     12px;
  --radius-sm:  8px;
}

/* ── Reset ──────────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
a { color: inherit; text-decoration: none; }

/* ── Base ───────────────────────────────────────────────────────────── */
body {
  background: var(--bg);
  color: var(--text);
  font-family: 'Nunito', sans-serif;
  min-height: 100vh;
  overflow-x: hidden;
  line-height: 1.6;
}
body::before {
  content: '';
  position: fixed;
  inset: 0;
  background-image:
    radial-gradient(circle at 15% 20%, rgba(240,165,0,0.04) 0%, transparent 50%),
    radial-gradient(circle at 85% 70%, rgba(57,197,207,0.04) 0%, transparent 50%);
  pointer-events: none;
  z-index: 0;
}

/* ── Layout ─────────────────────────────────────────────────────────── */
.wrapper { position: relative; z-index: 1; max-width: 1100px; margin: 0 auto; padding: 0 24px 80px; }

/* ── Shared nav ─────────────────────────────────────────────────────── */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(13,17,23,0.92);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
  padding: 0 24px;
}
.nav-inner {
  max-width: 1100px;
  margin: 0 auto;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
}
.nav-logo {
  font-family: 'DM Serif Display', serif;
  font-size: 18px;
  color: var(--text);
}
.nav-logo span { color: var(--amber); }
.nav-back {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 13px;
  font-weight: 700;
  color: var(--text2);
  transition: color 0.15s;
  cursor: pointer;
  background: none;
  border: none;
  font-family: 'Nunito', sans-serif;
}
.nav-back:hover { color: var(--amber); }

/* ── Eyebrow label ──────────────────────────────────────────────────── */
.eyebrow {
  font-family: 'DM Mono', monospace;
  font-size: 11px;
  color: var(--amber);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  margin-bottom: 8px;
}

/* ── Tabs ───────────────────────────────────────────────────────────── */
.tabs {
  display: flex;
  gap: 4px;
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 4px;
  margin-bottom: 28px;
  overflow-x: auto;
}
.tab-btn {
  flex: 1;
  min-width: 120px;
  padding: 10px 16px;
  background: transparent;
  border: none;
  border-radius: var(--radius-sm);
  color: var(--text2);
  font-family: 'Nunito', sans-serif;
  font-size: 13px;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.2s;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 7px;
  white-space: nowrap;
}
.tab-btn:hover { color: var(--text); background: var(--bg3); }
.tab-btn.active { background: var(--amber); color: #0d1117; }
.tab-icon { font-size: 15px; }

/* ── Panels ─────────────────────────────────────────────────────────── */
.panel { display: none; animation: fadeIn 0.3s ease; }
.panel.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: translateY(0); } }

/* ── Cards ──────────────────────────────────────────────────────────── */
.card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
}

/* ── Badges ─────────────────────────────────────────────────────────── */
.badge {
  display: inline-flex;
  align-items: center;
  padding: 4px 12px;
  border-radius: 20px;
  font-family: 'DM Mono', monospace;
  font-size: 11px;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}
.badge-amber  { background: var(--amber-dim);  color: var(--amber);  border: 1px solid rgba(240,165,0,0.3); }
.badge-teal   { background: var(--teal-dim);   color: var(--teal);   border: 1px solid rgba(57,197,207,0.3); }
.badge-blue   { background: var(--blue-dim);   color: var(--blue);   border: 1px solid rgba(88,166,255,0.3); }
.badge-green  { background: var(--green-dim);  color: var(--green);  border: 1px solid rgba(63,185,80,0.3); }
.badge-red    { background: var(--red-dim);    color: var(--red);    border: 1px solid rgba(248,81,73,0.3); }

/* ── Buttons ─────────────────────────────────────────────────────────── */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  padding: 10px 20px;
  border-radius: var(--radius-sm);
  font-family: 'Nunito', sans-serif;
  font-size: 14px;
  font-weight: 700;
  cursor: pointer;
  transition: all 0.15s;
  border: none;
}
.btn-primary { background: var(--amber); color: #0d1117; }
.btn-primary:hover { background: var(--amber2); }
.btn-ghost { background: transparent; color: var(--text2); border: 1px solid var(--border); }
.btn-ghost:hover { border-color: var(--amber); color: var(--amber); background: var(--amber-dim); }

/* ── Tables ─────────────────────────────────────────────────────────── */
.data-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 13px;
}
.data-table th {
  background: var(--bg);
  color: var(--text2);
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid var(--border);
}
.data-table td {
  padding: 10px 14px;
  border-bottom: 1px solid var(--border);
  color: var(--text2);
  line-height: 1.5;
}
.data-table td:first-child { color: var(--text); font-weight: 600; }
.data-table tr:hover td { background: var(--bg3); }
.mono { font-family: 'DM Mono', monospace; font-size: 12px; color: var(--amber); }

/* ── Study guide ─────────────────────────────────────────────────────── */
.study-grid {
  display: grid;
  grid-template-columns: 240px 1fr;
  gap: 20px;
  align-items: start;
}
@media(max-width:700px) { .study-grid { grid-template-columns: 1fr; } }

.toc-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px;
  position: sticky;
  top: 76px;
}
.toc-title {
  font-size: 10px;
  font-weight: 800;
  color: var(--text3);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  margin-bottom: 12px;
}
.toc-item {
  display: block;
  padding: 8px 10px;
  border-radius: var(--radius-sm);
  font-size: 13px;
  color: var(--text2);
  cursor: pointer;
  transition: all 0.15s;
  border: none;
  background: none;
  width: 100%;
  text-align: left;
  font-family: 'Nunito', sans-serif;
  font-weight: 600;
  margin-bottom: 2px;
}
.toc-item:hover { background: var(--bg3); color: var(--text); }
.toc-item.active { background: var(--amber-dim); color: var(--amber); border-left: 3px solid var(--amber); padding-left: 13px; }

.content-card { background: var(--bg2); border: 1px solid var(--border); border-radius: var(--radius); padding: 32px; }
.content-section { display: none; }
.content-section.active { display: block; animation: fadeIn 0.25s ease; }
.content-section h2 { font-family: 'DM Serif Display', serif; font-size: 26px; color: var(--text); margin-bottom: 16px; }
.section-body { font-size: 15px; line-height: 1.75; color: var(--text2); margin-bottom: 20px; }

.key-point {
  background: var(--amber-dim);
  border-left: 3px solid var(--amber);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: 14px 18px;
  margin: 16px 0;
  font-size: 14px;
  color: var(--text);
  line-height: 1.6;
}

/* ── Flashcards ──────────────────────────────────────────────────────── */
.fc-controls { display: flex; gap: 12px; align-items: center; margin-bottom: 28px; flex-wrap: wrap; }
.filter-btns { display: flex; gap: 6px; flex-wrap: wrap; }
.filter-btn {
  padding: 6px 14px;
  border-radius: 20px;
  border: 1px solid var(--border);
  background: transparent;
  color: var(--text2);
  font-size: 12px;
  font-weight: 700;
  font-family: 'Nunito', sans-serif;
  cursor: pointer;
  transition: all 0.15s;
}
.filter-btn:hover { border-color: var(--amber); color: var(--amber); }
.filter-btn.active { background: var(--amber); border-color: var(--amber); color: #0d1117; }

.fc-stage { perspective: 1200px; margin-bottom: 24px; }
.fc-card-wrap {
  position: relative;
  height: 280px;
  cursor: pointer;
  transform-style: preserve-3d;
  transition: transform 0.55s cubic-bezier(0.4,0,0.2,1);
  user-select: none;
}
.fc-card-wrap.flipped { transform: rotateY(180deg); }
.fc-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  border-radius: var(--radius);
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  padding: 36px 40px;
  text-align: center;
}
.fc-front { background: var(--bg2); border: 1px solid var(--border2); }
.fc-back  { background: var(--bg3); border: 1px solid var(--amber); transform: rotateY(180deg); }

.fc-tag {
  position: absolute;
  top: 16px; left: 16px;
  font-family: 'DM Mono', monospace;
  font-size: 10px;
  font-weight: 500;
  padding: 3px 10px;
  border-radius: 20px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
}
.tag-definition { background: var(--teal-dim);  color: var(--teal); }
.tag-formula    { background: var(--amber-dim); color: var(--amber); }
.tag-concept    { background: var(--blue-dim);  color: var(--blue); }

.fc-counter { position: absolute; top: 16px; right: 16px; font-family: 'DM Mono', monospace; font-size: 11px; color: var(--text3); }
.fc-question { font-family: 'DM Serif Display', serif; font-size: clamp(18px,3vw,24px); color: var(--text); line-height: 1.4; margin-bottom: 12px; }
.fc-hint     { font-size: 12px; color: var(--text3); margin-top: 16px; }
.fc-answer   { font-size: 15px; color: var(--text); line-height: 1.65; }
.fc-formula-display { font-family: 'DM Mono', monospace; font-size: 18px; color: var(--amber); background: var(--bg); padding: 8px 20px; border-radius: var(--radius-sm); margin: 12px 0; }

.fc-nav { display: flex; align-items: center; justify-content: center; gap: 16px; margin-bottom: 20px; }
.nav-btn {
  width: 44px; height: 44px;
  border-radius: 50%;
  background: var(--bg2);
  border: 1px solid var(--border);
  color: var(--text2);
  font-size: 20px;
  cursor: pointer;
  transition: all 0.15s;
  display: flex; align-items: center; justify-content: center;
}
.nav-btn:hover { border-color: var(--amber); color: var(--amber); background: var(--amber-dim); }
.nav-btn:disabled { opacity: 0.3; cursor: not-allowed; }
.fc-index { font-family: 'DM Mono', monospace; font-size: 13px; color: var(--text2); min-width: 60px; text-align: center; }

.fc-rating { display: flex; gap: 10px; justify-content: center; margin-bottom: 20px; }
.rate-btn {
  padding: 8px 22px;
  border-radius: 20px;
  border: 1px solid;
  font-size: 13px;
  font-weight: 700;
  font-family: 'Nunito', sans-serif;
  cursor: pointer;
  transition: all 0.15s;
  display: flex; align-items: center; gap: 6px;
}
.rate-easy { border-color: rgba(63,185,80,0.4); color: var(--green); background: var(--green-dim); }
.rate-easy:hover { background: var(--green); color: #0d1117; }
.rate-hard { border-color: rgba(248,81,73,0.4); color: var(--red); background: var(--red-dim); }
.rate-hard:hover { background: var(--red); color: #fff; }

.fc-dots { display: flex; justify-content: center; gap: 6px; margin-top: 16px; flex-wrap: wrap; }
.fc-dot { width: 8px; height: 8px; border-radius: 50%; background: var(--bg4); cursor: pointer; transition: all 0.15s; }
.fc-dot.seen   { background: var(--border2); }
.fc-dot.easy   { background: var(--green); }
.fc-dot.hard   { background: var(--red); }
.fc-dot.current { background: var(--amber); transform: scale(1.4); }

/* ── Practice paper ──────────────────────────────────────────────────── */
.paper-header {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px 24px;
  margin-bottom: 24px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
}
.paper-meta { font-size: 13px; color: var(--text2); }
.paper-meta strong { color: var(--text); }
.score-display { font-family: 'DM Serif Display', serif; font-size: 28px; color: var(--amber); }
.score-label { font-size: 12px; color: var(--text2); text-align: right; }

.q-section-title {
  font-size: 11px;
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: 0.14em;
  color: var(--text3);
  margin: 28px 0 14px;
  display: flex;
  align-items: center;
  gap: 10px;
}
.q-section-title::after { content: ''; flex: 1; height: 1px; background: var(--border); }

.q-card {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px 24px;
  margin-bottom: 14px;
  transition: border-color 0.2s;
}
.q-card.correct { border-color: var(--green); }
.q-card.wrong   { border-color: var(--red); }

.q-num {
  font-family: 'DM Mono', monospace;
  font-size: 11px;
  color: var(--text3);
  margin-bottom: 10px;
}
.q-text {
  font-size: 15px;
  color: var(--text);
  line-height: 1.6;
  margin-bottom: 16px;
}
.options-grid { display: grid; gap: 8px; }
.option-btn {
  display: flex;
  align-items: flex-start;
  gap: 12px;
  padding: 12px 16px;
  background: var(--bg3);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: 14px;
  color: var(--text2);
  cursor: pointer;
  transition: all 0.15s;
  text-align: left;
  font-family: 'Nunito', sans-serif;
  width: 100%;
}
.option-btn:hover:not(:disabled) { border-color: var(--amber); color: var(--text); background: var(--amber-dim); }
.option-btn .opt-letter {
  flex-shrink: 0;
  width: 22px; height: 22px;
  border-radius: 50%;
  background: var(--bg4);
  font-size: 11px;
  font-weight: 800;
  display: flex; align-items: center; justify-content: center;
  color: var(--text2);
  font-family: 'DM Mono', monospace;
}
.option-btn.selected { border-color: var(--amber); background: var(--amber-dim); color: var(--text); }
.option-btn.selected .opt-letter { background: var(--amber); color: #0d1117; }
.option-btn.correct-ans { border-color: var(--green); background: var(--green-dim); color: var(--text); }
.option-btn.correct-ans .opt-letter { background: var(--green); color: #0d1117; }
.option-btn.wrong-ans { border-color: var(--red); background: var(--red-dim); }

.explanation {
  display: none;
  margin-top: 12px;
  padding: 12px 16px;
  background: var(--bg3);
  border-left: 3px solid var(--green);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 13px;
  color: var(--text2);
  line-height: 1.6;
}
.explanation.show { display: block; animation: fadeIn 0.25s ease; }

.sa-card { background: var(--bg2); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px 24px; margin-bottom: 14px; }
.sa-marks { font-family: 'DM Mono', monospace; font-size: 11px; color: var(--text3); margin-bottom: 8px; }
.sa-q { font-size: 15px; color: var(--text); margin-bottom: 14px; line-height: 1.6; }
.reveal-btn {
  padding: 7px 16px;
  border-radius: var(--radius-sm);
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text2);
  font-size: 13px;
  font-weight: 700;
  font-family: 'Nunito', sans-serif;
  cursor: pointer;
  transition: all 0.15s;
}
.reveal-btn:hover { border-color: var(--teal); color: var(--teal); }
.model-answer {
  display: none;
  margin-top: 14px;
  padding: 16px 18px;
  background: var(--bg3);
  border-left: 3px solid var(--teal);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 14px;
  color: var(--text2);
  line-height: 1.7;
}
.model-answer.show { display: block; animation: fadeIn 0.25s ease; }
.model-label { font-size: 10px; font-weight: 800; text-transform: uppercase; letter-spacing: 0.1em; color: var(--teal); margin-bottom: 8px; }

/* ── Formula sheet ───────────────────────────────────────────────────── */
.formula-card { background: var(--bg2); border: 1px solid var(--border); border-radius: var(--radius); overflow: hidden; margin-bottom: 16px; }
.formula-card-header { padding: 12px 20px; background: var(--bg); font-size: 11px; font-weight: 800; text-transform: uppercase; letter-spacing: 0.12em; color: var(--text3); border-bottom: 1px solid var(--border); }

/* ── Loading / error states ─────────────────────────────────────────── */
.loading-state {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 300px;
  gap: 16px;
  color: var(--text2);
}
.spinner {
  width: 36px; height: 36px;
  border: 3px solid var(--border);
  border-top-color: var(--amber);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }
.error-state {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 300px;
  gap: 12px;
  color: var(--red);
  text-align: center;
}