/* ================================================================
   Euclides — Design System
   Identity: mathematical precision, scholarly warmth.
   No gradients, no rounded excess. Grid-based, deliberate.
   ================================================================ */

/* ── Tokens ─────────────────────────────────────────────────────── */
:root {
  --bg:          #0c0c10;
  --surface:     #13131c;
  --surface-2:   #1a1a28;
  --border:      #22223a;
  --border-2:    #2e2e4a;

  --text:        #e8e8f2;
  --text-muted:  #72728c;
  --text-dim:    #3e3e58;

  --gold:        #c8a44a;    /* primary accent */
  --gold-light:  #e0bb6a;
  --gold-dim:    rgba(200,164,74,.15);

  --teal:        #4ab8b2;    /* canvas / interactive */
  --blue:        #5a8fd2;    /* secondary actions */
  --green:       #4e9e70;    /* correct */
  --red:         #c45252;    /* wrong */

  --sidebar-w:   220px;
  --radius:      4px;
  --gap:         24px;

  --font-ui:     'Inter', system-ui, sans-serif;
  --font-mono:   'JetBrains Mono', 'Fira Code', monospace;
}

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

html, body { height: 100%; overflow: hidden; }

body {
  font-family: var(--font-ui);
  font-size: 14px;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

/* ── App shell ──────────────────────────────────────────────────── */
/* ── Top bar ────────────────────────────────────────────────────── */
#topbar {
  height: 44px;
  flex-shrink: 0;
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 16px;
  position: relative;
  z-index: 200;
}

#topbar-brand {
  display: flex;
  align-items: center;
  gap: 10px;
}

#topbar-brand .logo-mark {
  width: 26px;
  height: 26px;
  flex-shrink: 0;
}

#topbar-brand .logo-text {
  font-size: 14px;
  font-weight: 600;
  letter-spacing: .02em;
  color: var(--text);
}

#topbar-nav {
  display: flex;
  gap: 2px;
}

.topbar-tab {
  background: transparent;
  border: none;
  color: var(--text-muted);
  font-family: var(--font-ui);
  font-size: 12px;
  font-weight: 500;
  padding: 5px 12px;
  border-radius: var(--radius);
  cursor: pointer;
  transition: color .15s, background .15s;
  text-decoration: none;
  display: inline-flex;
  align-items: center;
}

.topbar-tab:hover {
  color: var(--text);
  background: var(--surface-2);
}

.topbar-tab.active {
  color: var(--gold);
  background: var(--gold-dim);
}

/* ── App shell (below topbar) ───────────────────────────────────── */
body {
  display: flex;
  flex-direction: column;
  height: 100vh;
  overflow: hidden;
}

#app {
  display: flex;
  flex: 1;
  overflow: hidden;
}

/* ── Sidebar ────────────────────────────────────────────────────── */
#sidebar {
  width: var(--sidebar-w);
  flex-shrink: 0;
  background: var(--surface);
  border-right: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  overflow: hidden;
}

#sidebar-header {
  display: none;   /* shown only on mobile via media query */
  align-items: center;
  justify-content: flex-end;
  padding: 10px 12px 8px;
  border-bottom: 1px solid var(--border);
  min-height: 36px;
}

/* logo styles defined inside #topbar-brand above */

#topic-nav {
  flex: 1;
  overflow-y: auto;
  padding: 8px 0;
  scrollbar-width: thin;
  scrollbar-color: var(--border-2) transparent;
}

.nav-section-label {
  font-size: 9px;
  font-weight: 700;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--text-dim);
  padding: 16px 16px 3px;
  border-top: 1px solid var(--border);
  margin-top: 4px;
}

.nav-section-label:first-child {
  border-top: none;
  margin-top: 0;
  padding-top: 12px;
}

.nav-subgroup-label {
  font-size: 9px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--gold);
  opacity: .55;
  padding: 8px 20px 2px;
}

.nav-item {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 16px;
  cursor: pointer;
  color: var(--text-muted);
  font-size: 13px;
  border: none;
  background: none;
  width: 100%;
  text-align: left;
  transition: color .15s, background .15s;
  border-left: 2px solid transparent;
}

.nav-item:hover {
  color: var(--text);
  background: var(--surface-2);
}

.nav-item.active {
  color: var(--gold);
  background: var(--gold-dim);
  border-left-color: var(--gold);
}

.nav-item.coming-soon {
  opacity: .4;
  cursor: not-allowed;
}

.nav-item.coming-soon:hover {
  color: var(--text-muted);
  background: none;
}

.nav-badge {
  margin-left: auto;
  font-size: 10px;
  font-family: var(--font-mono);
  background: var(--surface-2);
  color: var(--text-dim);
  padding: 1px 5px;
  border-radius: 2px;
}

.nav-item.active .nav-badge,
.nav-item:hover .nav-badge {
  background: var(--gold-dim);
  color: var(--gold);
}

#sidebar-footer {
  padding: 12px;
  border-top: 1px solid var(--border);
}

#btn-teacher {
  width: 100%;
  padding: 8px;
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-radius: var(--radius);
  color: var(--text-muted);
  font-family: var(--font-ui);
  font-size: 12px;
  cursor: pointer;
  transition: color .15s, border-color .15s;
}

#btn-teacher:hover {
  color: var(--text);
  border-color: var(--gold);
}

/* ── Top panel modal ─────────────────────────────────────────────── */
#tpanel-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,.5);
  z-index: 500;
}

#tpanel {
  display: none;
  position: fixed;
  top: 44px;
  right: 0;
  width: 320px;
  max-height: calc(100vh - 44px);
  background: var(--surface);
  border-left: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
  z-index: 501;
  flex-direction: column;
  overflow: hidden;
}

#tpanel.open,
#tpanel-overlay.open {
  display: flex;
}

#tpanel-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 16px 12px;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

#tpanel-title {
  font-family: var(--font-ui);
  font-size: 13px;
  font-weight: 600;
  color: var(--text);
}

#tpanel-close {
  background: transparent;
  border: none;
  color: var(--text-muted);
  font-size: 18px;
  line-height: 1;
  cursor: pointer;
  padding: 0 2px;
  transition: color .15s;
}

#tpanel-close:hover { color: var(--text); }

#tpanel-body {
  padding: 20px 16px;
  overflow-y: auto;
  flex: 1;
  font-family: var(--font-ui);
  font-size: 13px;
  color: var(--text-muted);
  line-height: 1.6;
}

#tpanel-body h3 {
  font-size: 11px;
  font-weight: 700;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--text-dim);
  margin: 16px 0 6px;
}

#tpanel-body h3:first-child { margin-top: 0; }

#tpanel-body p { margin: 0 0 10px; }

#tpanel-body a {
  color: var(--blue);
  text-decoration: none;
}

#tpanel-body a:hover { text-decoration: underline; }

.eco-link {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 9px 10px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  background: var(--surface-2);
  margin-bottom: 6px;
  text-decoration: none !important;
  transition: border-color .15s, background .15s;
}

.eco-link:hover {
  border-color: var(--gold) !important;
  background: var(--gold-dim) !important;
}

.eco-link-name {
  font-size: 13px;
  font-weight: 500;
  color: var(--text);
}

.eco-link-desc {
  font-size: 11px;
  color: var(--text-muted);
  margin-top: 1px;
}

.help-step {
  display: flex;
  gap: 10px;
  margin-bottom: 12px;
}

.help-num {
  font-family: var(--font-mono);
  font-size: 11px;
  font-weight: 600;
  color: var(--gold);
  background: var(--gold-dim);
  border-radius: 50%;
  width: 20px;
  height: 20px;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
  margin-top: 1px;
}

/* ── Main content ───────────────────────────────────────────────── */
#main {
  flex: 1;
  overflow: hidden;
  display: flex;
  flex-direction: column;
}

#view {
  flex: 1;
  overflow-y: auto;
  scrollbar-width: thin;
  scrollbar-color: var(--border-2) transparent;
}

/* ── Home screen ────────────────────────────────────────────────── */
.home-screen {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 100%;
  gap: 32px;
  padding: var(--gap);
}

.home-icon {
  width: 96px;
  height: 96px;
  border-radius: 22px;
  box-shadow: 0 8px 40px rgba(200,164,74,0.15), 0 2px 12px rgba(0,0,0,0.5);
}

.home-title {
  font-family: var(--font-mono);
  font-size: 48px;
  font-weight: 400;
  color: var(--text);
  letter-spacing: -.02em;
}

.home-subtitle {
  font-size: 16px;
  color: var(--text-muted);
  text-align: center;
  max-width: 480px;
  line-height: 1.7;
}

.home-prompt {
  font-size: 13px;
  color: var(--text-dim);
}

/* ── Progress bar (sidebar) ─────────────────────────────────────── */
#progress-bar-wrap {
  padding: 12px 16px 8px;
  border-bottom: 1px solid var(--border);
}

.prog-bar-row {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  margin-bottom: 6px;
}

.prog-label {
  font-size: 10px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--text-dim);
}

.prog-value {
  font-family: var(--font-mono);
  font-size: 11px;
  color: var(--gold);
}

.prog-bar-track {
  height: 4px;
  background: var(--surface-2);
  border-radius: 2px;
  overflow: hidden;
}

.prog-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--teal), var(--gold));
  border-radius: 2px;
  transition: width .4s ease;
  min-width: 0%;
}

/* ── Review button ──────────────────────────────────────────────── */
#btn-review {
  width: 100%;
  padding: 8px;
  background: rgba(93,232,224,0.07);
  border: 1px solid rgba(93,232,224,0.2);
  border-radius: var(--radius);
  color: var(--teal);
  font-family: var(--font-ui);
  font-size: 12px;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 6px;
  transition: background .15s, border-color .15s;
}

#btn-review:hover {
  background: rgba(93,232,224,0.14);
  border-color: var(--teal);
}

/* ── Review screen ──────────────────────────────────────────────── */
.review-screen {
  display: flex;
  flex-direction: column;
  height: 100%;
  padding: 32px;
  gap: 24px;
}

.review-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-shrink: 0;
}

.review-meta {
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.review-label {
  font-size: 10px;
  font-weight: 600;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--text-dim);
}

.review-topic {
  font-family: var(--font-mono);
  font-size: 15px;
  font-weight: 500;
}

.review-stats {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 13px;
  color: var(--text-muted);
}

.stat-val {
  font-family: var(--font-mono);
  font-size: 18px;
  font-weight: 600;
  color: var(--text);
}

.stat-sep { color: var(--text-dim); }

.streak-badge {
  font-size: 13px;
  color: var(--gold);
  margin-left: 8px;
}

.review-body {
  flex: 1;
  overflow-y: auto;
  display: flex;
  align-items: flex-start;
  justify-content: center;
}

.review-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-shrink: 0;
  padding-top: 8px;
  border-top: 1px solid var(--border);
}

.review-empty {
  text-align: center;
  padding: 64px 32px;
  color: var(--text-muted);
}

/* ── Correct answer flash animation ────────────────────────────── */
@keyframes flashCorrect {
  0%   { text-shadow: 0 0 0 transparent; }
  20%  { text-shadow: 0 0 12px rgba(93,232,224,0.8); color: var(--teal); }
  100% { text-shadow: none; }
}

.feedback-line.anim-flash {
  animation: flashCorrect .6s ease-out forwards;
}


.topic-screen {
  display: grid;
  grid-template-columns: 1fr 420px;
  height: 100%;
  overflow: hidden;
}

.topic-screen.no-canvas {
  grid-template-columns: 1fr;
}

.topic-content {
  overflow-y: auto;
  padding: var(--gap) 32px;
  scrollbar-width: thin;
  scrollbar-color: var(--border-2) transparent;
}

.topic-canvas-panel {
  border-left: 1px solid var(--border);
  background: var(--surface);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 16px;
  gap: 16px;
}

/* ── Breadcrumb ─────────────────────────────────────────────────── */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 12px;
  color: var(--text-dim);
  margin-bottom: 28px;
}

.breadcrumb span { cursor: pointer; transition: color .15s; }
.breadcrumb span:hover { color: var(--text-muted); }
.breadcrumb .sep { color: var(--text-dim); }
.breadcrumb .current { color: var(--text-muted); cursor: default; }

/* ── Phase header ───────────────────────────────────────────────── */
.phase-bar {
  display: flex;
  align-items: center;
  gap: 0;
  margin-bottom: 32px;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  width: fit-content;
}

.phase-step {
  padding: 6px 16px;
  font-size: 12px;
  font-weight: 500;
  color: var(--text-dim);
  background: var(--surface);
  border-right: 1px solid var(--border);
  cursor: pointer;
  transition: color .15s, background .15s;
}

.phase-step:last-child { border-right: none; }
.phase-step:hover { color: var(--text-muted); background: var(--surface-2); }

.phase-step.active {
  color: var(--gold);
  background: var(--gold-dim);
}

.phase-step.done {
  color: var(--green);
}

/* ── Content typography ─────────────────────────────────────────── */
.topic-title {
  font-size: 26px;
  font-weight: 600;
  color: var(--text);
  margin-bottom: 8px;
  line-height: 1.25;
}

.topic-meta {
  font-size: 12px;
  color: var(--text-dim);
  font-family: var(--font-mono);
  margin-bottom: 28px;
}

.content-block {
  margin-bottom: 24px;
  max-width: 640px;
}

.content-block p {
  color: var(--text-muted);
  line-height: 1.75;
  margin-bottom: 14px;
}

.content-block p:last-child { margin-bottom: 0; }

.concept-highlight {
  background: var(--surface-2);
  border-left: 3px solid var(--gold);
  padding: 14px 18px;
  border-radius: 0 var(--radius) var(--radius) 0;
  margin: 20px 0;
  font-family: var(--font-mono);
  font-size: 14px;
  color: var(--text);
  line-height: 1.8;
}

.concept-highlight .hl-label {
  font-size: 10px;
  font-weight: 600;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--gold);
  margin-bottom: 6px;
}

/* ── Example step display ───────────────────────────────────────── */
.example-step-bar {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 24px;
}

.step-counter {
  font-family: var(--font-mono);
  font-size: 12px;
  color: var(--text-dim);
}

.step-progress {
  flex: 1;
  height: 2px;
  background: var(--border);
  border-radius: 1px;
  overflow: hidden;
}

.step-progress-fill {
  height: 100%;
  background: var(--gold);
  transition: width .3s ease;
}

.step-description {
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-radius: var(--radius);
  padding: 14px 18px;
  margin-bottom: 24px;
  font-size: 14px;
  color: var(--text-muted);
  max-width: 640px;
  min-height: 52px;
}

.step-description strong { color: var(--text); font-weight: 500; }

/* ── Practice area ──────────────────────────────────────────────── */
.practice-header {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  margin-bottom: 24px;
  max-width: 640px;
}

.exercise-counter {
  font-family: var(--font-mono);
  font-size: 12px;
  color: var(--text-dim);
}

.hint-toggle {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 12px;
  color: var(--text-dim);
  cursor: pointer;
  user-select: none;
}

.toggle-switch {
  width: 30px;
  height: 16px;
  background: var(--border-2);
  border-radius: 8px;
  position: relative;
  transition: background .2s;
}

.toggle-switch.on { background: var(--gold); }

.toggle-knob {
  position: absolute;
  top: 2px; left: 2px;
  width: 12px; height: 12px;
  background: var(--text);
  border-radius: 50%;
  transition: left .2s;
}

.toggle-switch.on .toggle-knob { left: 16px; }

.exercise-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  max-width: 640px;
  margin-bottom: 20px;
}

.exercise-statement {
  font-size: 15px;
  color: var(--text);
  line-height: 1.65;
  margin-bottom: 20px;
}

.exercise-equation {
  font-family: var(--font-mono);
  font-size: 20px;
  color: var(--gold);
  text-align: center;
  padding: 16px;
  background: var(--surface-2);
  border-radius: var(--radius);
  margin-bottom: 24px;
  letter-spacing: .03em;
}

.answer-row {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 16px;
}

.answer-label {
  font-family: var(--font-mono);
  font-size: 14px;
  color: var(--text-muted);
  white-space: nowrap;
}

.answer-input {
  flex: 1;
  max-width: 220px;
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-radius: var(--radius);
  padding: 9px 14px;
  font-family: var(--font-mono);
  font-size: 16px;
  color: var(--text);
  outline: none;
  transition: border-color .15s;
}

.answer-input:focus { border-color: var(--gold); }
.answer-input.correct { border-color: var(--green); }
.answer-input.wrong   { border-color: var(--red); }

.feedback-line {
  font-size: 13px;
  min-height: 20px;
  margin-bottom: 16px;
  font-family: var(--font-mono);
}

.feedback-line.correct { color: var(--green); }
.feedback-line.wrong   { color: var(--red); }

.hint-box {
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-left: 3px solid var(--teal);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 12px 16px;
  font-size: 13px;
  color: var(--text-muted);
  margin-bottom: 16px;
  line-height: 1.65;
  max-width: 580px;
}

.hint-box .hint-label {
  font-size: 10px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--teal);
  margin-bottom: 5px;
}

/* ── Buttons ────────────────────────────────────────────────────── */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 9px 20px;
  border: 1px solid var(--border-2);
  border-radius: var(--radius);
  background: var(--surface-2);
  color: var(--text-muted);
  font-family: var(--font-ui);
  font-size: 13px;
  font-weight: 500;
  cursor: pointer;
  transition: color .15s, border-color .15s, background .15s;
  text-decoration: none;
}

.btn:hover { color: var(--text); border-color: var(--text-dim); }

.btn-primary {
  background: var(--gold-dim);
  border-color: var(--gold);
  color: var(--gold-light);
}

.btn-primary:hover {
  background: rgba(200,164,74,.25);
  color: var(--gold-light);
}

.btn-sm {
  padding: 6px 14px;
  font-size: 12px;
}

.btn-row {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
}

/* ── Canvas ─────────────────────────────────────────────────────── */
#main-canvas {
  display: block;
  border-radius: var(--radius);
  border: 1px solid var(--border);
}

/* ── Teacher mode ───────────────────────────────────────────────── */
.teacher-screen {
  padding: var(--gap) 32px;
  max-width: 760px;
}

.teacher-title {
  font-size: 22px;
  font-weight: 600;
  color: var(--text);
  margin-bottom: 6px;
}

.teacher-desc {
  font-size: 13px;
  color: var(--text-muted);
  margin-bottom: 32px;
}

.form-group {
  margin-bottom: 20px;
}

.form-label {
  display: block;
  font-size: 12px;
  font-weight: 500;
  color: var(--text-muted);
  margin-bottom: 6px;
  text-transform: uppercase;
  letter-spacing: .06em;
}

.form-input, .form-select, .form-textarea {
  width: 100%;
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-radius: var(--radius);
  padding: 9px 14px;
  font-family: var(--font-ui);
  font-size: 14px;
  color: var(--text);
  outline: none;
  transition: border-color .15s;
}

.form-input:focus, .form-select:focus, .form-textarea:focus {
  border-color: var(--gold);
}

.form-select { cursor: pointer; }

.form-textarea {
  resize: vertical;
  min-height: 80px;
  font-family: var(--font-mono);
  font-size: 13px;
}

.hint-fields { display: flex; flex-direction: column; gap: 8px; }

/* ── Exercise list (teacher) ────────────────────────────────────── */
.exercise-list {
  margin-top: 40px;
}

.exercise-list-title {
  font-size: 13px;
  font-weight: 600;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: .08em;
  margin-bottom: 14px;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--border);
}

.exercise-item {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 14px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: 8px;
  gap: 12px;
}

.exercise-item-info { flex: 1; min-width: 0; }

.exercise-item-topic {
  font-size: 10px;
  font-family: var(--font-mono);
  color: var(--gold);
  text-transform: uppercase;
  letter-spacing: .06em;
  margin-bottom: 2px;
}

.exercise-item-stmt {
  font-size: 13px;
  color: var(--text);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.btn-icon {
  background: none;
  border: none;
  color: var(--text-dim);
  cursor: pointer;
  padding: 4px;
  font-size: 14px;
  line-height: 1;
  transition: color .15s;
  flex-shrink: 0;
}

.btn-icon:hover { color: var(--red); }

/* ── Divider ─────────────────────────────────────────────────────── */
hr.section-divider {
  border: none;
  border-top: 1px solid var(--border);
  margin: 32px 0;
}

/* ── Util ────────────────────────────────────────────────────────── */
.sr-only { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0,0,0,0); }
.text-mono { font-family: var(--font-mono); }
.text-gold  { color: var(--gold); }
.text-muted { color: var(--text-muted); }
.text-dim   { color: var(--text-dim); }
.mt-8  { margin-top: 8px; }
.mt-16 { margin-top: 16px; }
.mt-24 { margin-top: 24px; }
.mb-0  { margin-bottom: 0 !important; }

/* ── Exam button ────────────────────────────────────────────────── */
#btn-exam {
  width: 100%;
  padding: 8px;
  background: rgba(200,164,74,0.08);
  border: 1px solid rgba(200,164,74,0.25);
  border-radius: var(--radius);
  color: var(--gold);
  font-family: var(--font-ui);
  font-size: 12px;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 6px;
  transition: background .15s, border-color .15s;
}
#btn-exam:hover {
  background: rgba(200,164,74,0.16);
  border-color: var(--gold);
}

/* ── Completion overlay ─────────────────────────────────────────── */
#completion-overlay {
  position: fixed;
  inset: 0;
  z-index: 10000;
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  transition: opacity .25s ease;
  pointer-events: none;
}
#completion-overlay.cpl-visible {
  opacity: 1;
  pointer-events: all;
}
#completion-overlay.cpl-hiding {
  opacity: 0;
  pointer-events: none;
}
.cpl-backdrop {
  position: absolute;
  inset: 0;
  background: rgba(5,5,14,.82);
  backdrop-filter: blur(4px);
}
.cpl-card {
  position: relative;
  z-index: 1;
  background: var(--surface);
  border: 1px solid var(--border-2);
  border-radius: 12px;
  padding: 40px 48px;
  max-width: 420px;
  width: 90%;
  text-align: center;
  box-shadow: 0 24px 80px rgba(0,0,0,.6), 0 0 0 1px rgba(200,164,74,.08);
  transform: translateY(12px);
  transition: transform .3s ease;
}
#completion-overlay.cpl-visible .cpl-card {
  transform: translateY(0);
}
.cpl-icon {
  margin-bottom: 20px;
  display: flex;
  justify-content: center;
}
.cpl-title {
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--text-dim);
  margin-bottom: 8px;
}
.cpl-topic {
  font-family: var(--font-mono);
  font-size: 22px;
  color: var(--gold);
  margin-bottom: 16px;
}
.cpl-body {
  font-size: 14px;
  color: var(--text-muted);
  line-height: 1.6;
  margin-bottom: 28px;
}
.cpl-actions {
  display: flex;
  gap: 10px;
  justify-content: center;
}

/* ── Exam screen ────────────────────────────────────────────────── */
.exam-screen {
  display: flex;
  flex-direction: column;
  height: 100%;
  padding: 28px 32px;
  gap: 20px;
}
.exam-results {
  align-items: center;
  justify-content: center;
}
.exam-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-end;
  flex-shrink: 0;
}
.exam-progress-wrap { flex: 1; margin-right: 32px; }
.exam-progress-row {
  display: flex;
  justify-content: space-between;
  margin-bottom: 6px;
}
.exam-label {
  font-size: 10px;
  font-weight: 600;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--text-dim);
}
.exam-counter {
  font-family: var(--font-mono);
  font-size: 12px;
  color: var(--text-muted);
}
.exam-prog-track {
  height: 4px;
  background: var(--surface-2);
  border-radius: 2px;
  overflow: hidden;
}
.exam-prog-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--blue), var(--teal));
  border-radius: 2px;
  transition: width .3s ease;
}
.exam-timer-wrap {
  display: flex;
  flex-direction: column;
  align-items: flex-end;
  gap: 2px;
}
.exam-timer-label {
  font-size: 10px;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--text-dim);
}
.exam-timer {
  font-family: var(--font-mono);
  font-size: 22px;
  font-weight: 600;
  color: var(--text);
  letter-spacing: .04em;
  transition: color .3s;
}
.exam-timer-urgent {
  color: var(--red) !important;
  animation: timerPulse .8s ease-in-out infinite;
}
@keyframes timerPulse {
  0%,100% { opacity: 1; }
  50%      { opacity: .5; }
}
.exam-body {
  flex: 1;
  overflow-y: auto;
  display: flex;
  align-items: flex-start;
  justify-content: center;
}
.exam-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-shrink: 0;
  padding-top: 12px;
  border-top: 1px solid var(--border);
}
.exam-score-live {
  font-family: var(--font-mono);
  font-size: 12px;
  color: var(--teal);
}
.exam-topic-tag {
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--text-dim);
  margin-bottom: 12px;
}

/* ── Exam results ───────────────────────────────────────────────── */
.result-card {
  background: var(--surface);
  border: 1px solid var(--border-2);
  border-radius: 12px;
  padding: 48px 56px;
  max-width: 440px;
  width: 90%;
  text-align: center;
  box-shadow: 0 24px 80px rgba(0,0,0,.5);
}
.result-score-ring {
  display: flex;
  justify-content: center;
  margin-bottom: 24px;
}
.result-title {
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .12em;
  text-transform: uppercase;
  color: var(--text-dim);
  margin-bottom: 28px;
}
.result-stats {
  display: flex;
  justify-content: center;
  gap: 36px;
  margin-bottom: 20px;
}
.result-stat {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
}
.result-stat-val {
  font-family: var(--font-mono);
  font-size: 28px;
  font-weight: 600;
  line-height: 1;
}
.result-stat-label {
  font-size: 11px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: .06em;
}
.result-time {
  font-size: 13px;
  color: var(--text-dim);
  font-family: var(--font-mono);
  margin-bottom: 0;
}

/* ── Teacher tabs ───────────────────────────────────────────────── */
.teacher-tabs {
  display: flex;
  gap: 2px;
  margin-bottom: 24px;
  border-bottom: 1px solid var(--border);
  padding-bottom: 0;
}

.t-tab {
  padding: 8px 18px;
  background: none;
  border: none;
  border-bottom: 2px solid transparent;
  color: var(--text-muted);
  font-family: var(--font-ui);
  font-size: 13px;
  font-weight: 500;
  cursor: pointer;
  margin-bottom: -1px;
  transition: color .15s, border-color .15s;
}

.t-tab:hover { color: var(--text); }

.t-tab-active {
  color: var(--gold);
  border-bottom-color: var(--gold);
}

/* ── Sheet controls ─────────────────────────────────────────────── */
.sheet-controls {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: 0 20px;
  align-items: end;
}

.sheet-controls .btn-row {
  grid-column: 1 / -1;
  margin-top: 4px;
}

/* ── Sheet preview (screen) ─────────────────────────────────────── */
.sh-sheet {
  margin-top: 28px;
  background: #fff;
  color: #111;
  border-radius: 4px;
  padding: 32px 40px;
  box-shadow: 0 4px 32px rgba(0,0,0,.4);
  font-family: Georgia, 'Times New Roman', serif;
  max-width: 780px;
}

.sh-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 24px;
  margin-bottom: 12px;
}

.sh-header-col {
  flex: 1;
}

.sh-header-right {
  text-align: right;
}

.sh-header-center {
  text-align: center;
  flex: 0 0 auto;
}

.sh-title {
  font-size: 20px;
  font-weight: bold;
  letter-spacing: .04em;
  color: #111;
}

.sh-subtitle {
  font-size: 13px;
  color: #444;
  margin-top: 2px;
}

.sh-field-line {
  border-bottom: 1px solid #999;
  height: 22px;
  margin-bottom: 4px;
}

.sh-field-label {
  font-size: 10px;
  color: #888;
  font-family: Arial, sans-serif;
  text-transform: uppercase;
  letter-spacing: .06em;
}

.sh-date {
  font-size: 11px;
  color: #888;
  font-family: Arial, sans-serif;
  margin-top: 6px;
}

.sh-divider {
  border: none;
  border-top: 1.5px solid #222;
  margin: 8px 0 20px;
}

.sh-exercises {
  display: flex;
  flex-direction: column;
  gap: 20px;
}

.sh-exercise {
  display: flex;
  gap: 12px;
  page-break-inside: avoid;
}

.sh-num {
  font-size: 15px;
  font-weight: bold;
  color: #222;
  min-width: 28px;
  padding-top: 1px;
  flex-shrink: 0;
  font-family: Arial, sans-serif;
}

.sh-diff-badge {
  display: inline-block;
  font-size: 9px;
  font-weight: normal;
  background: #eee;
  color: #555;
  border-radius: 3px;
  padding: 1px 5px;
  vertical-align: middle;
  font-family: Arial, sans-serif;
  margin-left: 3px;
}

.sh-body {
  flex: 1;
}

.sh-statement {
  font-size: 13.5px;
  line-height: 1.55;
  color: #111;
  margin: 0 0 6px;
}

.sh-equation {
  font-family: 'Courier New', monospace;
  font-size: 13px;
  color: #333;
  margin-bottom: 8px;
  padding: 4px 8px;
  background: #f5f5f5;
  border-left: 3px solid #bbb;
  display: inline-block;
}

.sh-answer-line {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-top: 6px;
}

.sh-answer-label {
  font-size: 12px;
  color: #555;
  font-family: Arial, sans-serif;
  flex-shrink: 0;
}

.sh-blank {
  flex: 1;
  border-bottom: 1px solid #aaa;
  height: 18px;
  max-width: 200px;
}

.sh-answer-key {
  margin-top: 32px;
  padding-top: 16px;
  border-top: 1px dashed #bbb;
  page-break-before: auto;
}

.sh-key-title {
  font-size: 12px;
  font-weight: bold;
  text-transform: uppercase;
  letter-spacing: .08em;
  color: #444;
  font-family: Arial, sans-serif;
  margin-bottom: 10px;
}

.sh-key-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 24px;
}

.sh-key-item {
  font-size: 12px;
  color: #333;
  font-family: 'Courier New', monospace;
  min-width: 80px;
}

.sh-footer {
  margin-top: 32px;
  font-size: 10px;
  color: #bbb;
  text-align: center;
  font-family: Arial, sans-serif;
  letter-spacing: .04em;
}

/* ── Print stylesheet ───────────────────────────────────────────── */
@media print {
  /* Hide everything except the sheet */
  body > * { display: none !important; }
  #app      { display: none !important; }

  .sh-sheet {
    display: block !important;
    position: fixed;
    inset: 0;
    margin: 0;
    padding: 18mm 20mm;
    box-shadow: none;
    border-radius: 0;
    background: #fff;
    color: #000;
    max-width: none;
    font-size: 12pt;
  }

  .sh-title    { font-size: 16pt; }
  .sh-subtitle { font-size: 11pt; }
  .sh-statement{ font-size: 11pt; }
  .sh-equation { font-size: 10.5pt; }

  .sh-exercise { page-break-inside: avoid; }
  .sh-answer-key { page-break-before: auto; }

  @page {
    size: A4 portrait;
    margin: 0;
  }
}

/* ── Mobile sidebar ─────────────────────────────────────────────── */

#btn-sidebar-close {
  display: none;
  background: none;
  border: none;
  color: var(--text-muted);
  font-size: 20px;
  line-height: 1;
  cursor: pointer;
  padding: 4px 6px;
  margin-left: auto;
}
#btn-sidebar-close:hover { color: var(--text); }

#mobile-bar {
  display: none;
  align-items: center;
  gap: 12px;
  padding: 10px 16px;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

#btn-hamburger {
  background: none;
  border: none;
  color: var(--text-muted);
  font-size: 18px;
  cursor: pointer;
  padding: 4px 6px;
  line-height: 1;
}
#btn-hamburger:hover { color: var(--text); }

.mobile-title {
  font-family: var(--font-mono);
  font-size: 15px;
  font-weight: 500;
  color: var(--text);
}

#sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,.55);
  z-index: 99;
}

@media (max-width: 640px) {
  :root { --sidebar-w: 260px; }

  #main {
    flex-direction: column;
  }

  #mobile-bar {
    display: flex;
  }

  #view {
    flex: 1;
    overflow-y: auto;
  }

  #topbar-nav { gap: 0; }
  .topbar-tab  { padding: 5px 8px; font-size: 11px; }

  #sidebar {
    position: fixed;
    left: 0;
    top: 44px;           /* below topbar */
    height: calc(100% - 44px);
    z-index: 100;
    transform: translateX(-100%);
    transition: transform .25s ease;
    box-shadow: 4px 0 24px rgba(0,0,0,.5);
  }

  #sidebar.sidebar-open {
    transform: translateX(0);
  }

  #sidebar-overlay.sidebar-open {
    display: block;
  }

  #sidebar-header {
    display: flex;
  }

  #btn-sidebar-close {
    display: block;
  }

  /* Compress topic screen canvas on mobile */
  .topic-screen {
    grid-template-columns: 1fr;
  }
  .topic-canvas-panel {
    display: none;
  }

  /* Compress exam/review headers */
  .exam-header,
  .review-header {
    flex-direction: column;
    gap: 12px;
  }

  .exam-timer-wrap {
    align-items: flex-start;
  }

  .result-card {
    padding: 32px 24px;
  }

  .result-stats {
    gap: 20px;
  }

  /* Sheet controls stack on mobile */
  .sheet-controls {
    grid-template-columns: 1fr;
  }
}

/* ── Stats button ───────────────────────────────────────────────── */
#btn-stats {
  width: 100%;
  padding: 8px;
  background: rgba(90,143,210,0.08);
  border: 1px solid rgba(90,143,210,0.2);
  border-radius: var(--radius);
  color: var(--blue);
  font-family: var(--font-ui);
  font-size: 12px;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 6px;
  transition: background .15s, border-color .15s;
}
#btn-stats:hover {
  background: rgba(90,143,210,0.16);
  border-color: var(--blue);
}

/* ── Stats screen ───────────────────────────────────────────────── */
.stats-screen {
  padding: 32px;
  overflow-y: auto;
  height: 100%;
  max-width: 900px;
}

.stats-title {
  font-family: var(--font-mono);
  font-size: 22px;
  font-weight: 500;
  color: var(--text);
  margin-bottom: 24px;
}

.stats-empty {
  color: var(--text-muted);
  line-height: 1.8;
}

/* Summary cards */
.stats-cards {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 32px;
}

.stat-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 16px 20px;
  min-width: 110px;
  flex: 1;
}

.stat-card-accent {
  border-color: var(--gold);
  background: var(--gold-dim);
}

.stat-card-val {
  font-family: var(--font-mono);
  font-size: 28px;
  font-weight: 600;
  color: var(--text);
  line-height: 1;
  margin-bottom: 6px;
}

.stat-card-accent .stat-card-val {
  color: var(--gold);
}

.stat-card-label {
  font-size: 11px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: .07em;
}

/* Section titles */
.stats-section-title {
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--text-dim);
  margin-bottom: 12px;
  display: flex;
  align-items: center;
  gap: 8px;
}

.stats-count-badge {
  background: var(--surface-2);
  color: var(--text-muted);
  border-radius: 10px;
  padding: 1px 8px;
  font-size: 11px;
  font-weight: 500;
}

/* Level grid */
.stats-level-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 32px;
}

.stats-level-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 14px 18px;
  flex: 1;
  min-width: 160px;
}

.stats-level-name {
  font-size: 12px;
  font-weight: 600;
  color: var(--text-muted);
  margin-bottom: 8px;
}

.stats-level-row {
  display: flex;
  align-items: baseline;
  gap: 6px;
  margin-bottom: 8px;
}

.stats-level-num {
  font-family: var(--font-mono);
  font-size: 18px;
  font-weight: 600;
  color: var(--text);
}

.stats-level-sub {
  font-size: 11px;
  color: var(--text-dim);
}

.stats-prog-track {
  height: 3px;
  background: var(--border-2);
  border-radius: 2px;
  margin-bottom: 6px;
  overflow: hidden;
}

.stats-prog-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--teal), var(--gold));
  border-radius: 2px;
  transition: width .4s ease;
}

.stats-level-acc {
  font-size: 11px;
  color: var(--text-dim);
  font-family: var(--font-mono);
}

/* Two-column topic lists */
.stats-two-col {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 24px;
  margin-bottom: 8px;
}

/* Topic rows */
.stats-topic-row {
  display: grid;
  grid-template-columns: 1fr 80px 36px 52px;
  align-items: center;
  gap: 10px;
  padding: 8px 10px;
  border-radius: var(--radius);
  cursor: pointer;
  transition: background .12s;
  margin-bottom: 2px;
}

.stats-topic-row:hover { background: var(--surface); }

.stats-topic-name {
  font-size: 13px;
  color: var(--text-muted);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.stats-topic-bar-wrap {
  height: 4px;
  background: var(--border-2);
  border-radius: 2px;
  overflow: hidden;
}

.stats-topic-bar {
  height: 100%;
  border-radius: 2px;
  transition: width .3s ease;
}

.stats-topic-acc {
  font-family: var(--font-mono);
  font-size: 12px;
  font-weight: 600;
  text-align: right;
}

.stats-topic-count {
  font-family: var(--font-mono);
  font-size: 11px;
  text-align: right;
}

.stats-full-table {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 8px 4px;
  margin-bottom: 24px;
}

.stats-full-table .stats-topic-row {
  padding: 7px 12px;
}

.stats-footer {
  padding-top: 8px;
  border-top: 1px solid var(--border);
  display: flex;
  justify-content: flex-end;
}

/* Reset confirmation overlay */
.btn-danger {
  background: rgba(196,82,82,0.12);
  border: 1px solid rgba(196,82,82,0.3);
  color: var(--red);
}
.btn-danger:hover {
  background: rgba(196,82,82,0.22);
  border-color: var(--red);
}

#reset-overlay {
  position: fixed;
  inset: 0;
  z-index: 10000;
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  transition: opacity .2s;
  pointer-events: none;
}
#reset-overlay.reset-visible {
  opacity: 1;
  pointer-events: all;
}
.reset-backdrop {
  position: absolute;
  inset: 0;
  background: rgba(5,5,14,.8);
  backdrop-filter: blur(3px);
}
.reset-card {
  position: relative;
  z-index: 1;
  background: var(--surface);
  border: 1px solid var(--border-2);
  border-radius: 8px;
  padding: 32px 40px;
  max-width: 380px;
  width: 90%;
  text-align: center;
  box-shadow: 0 16px 60px rgba(0,0,0,.5);
}
.reset-title {
  font-family: var(--font-mono);
  font-size: 16px;
  color: var(--text);
  margin-bottom: 12px;
}
.reset-body {
  font-size: 13px;
  color: var(--text-muted);
  line-height: 1.6;
}

/* ── Mobile polish — refined breakpoint ─────────────────────────── */
@media (max-width: 640px) {
  /* Top panel full-width on mobile */
  #tpanel { width: 100%; border-left: none; }

  /* Tighter content padding */
  .topic-content {
    padding: 16px 18px;
  }

  /* Home screen compact */
  .home-screen { gap: 20px; padding: 24px 20px; }
  .home-title  { font-size: 32px; }
  .home-icon   { width: 72px; height: 72px; }

  /* Phase bar wrapping fix */
  .phase-bar { gap: 4px; }
  .phase-step { padding: 6px 10px; font-size: 12px; }

  /* Breadcrumb overflow */
  .breadcrumb { flex-wrap: wrap; }

  /* Exercise card full width */
  .exercise-card { max-width: 100% !important; }
  .answer-row { flex-wrap: wrap; gap: 8px; }
  .answer-input { min-width: 120px; }

  /* Hint boxes */
  .hint-box { padding: 10px 12px; }

  /* Topic title smaller */
  .topic-title { font-size: 20px; }

  /* Completion card */
  .cpl-card { padding: 28px 24px; }
  .cpl-topic { font-size: 18px; }
  .cpl-actions { flex-direction: column; }

  /* Stats screen */
  .stats-screen   { padding: 20px 16px; }
  .stats-two-col  { grid-template-columns: 1fr; }
  .stats-topic-row {
    grid-template-columns: 1fr 60px 36px;
  }
  .stats-topic-count { display: none; }

  /* Teacher screen */
  .teacher-screen { padding: 16px 18px; }

  /* Review/exam body padding */
  .review-screen,
  .exam-screen    { padding: 20px 16px; gap: 16px; }

  /* Sidebar footer buttons full width */
  #sidebar-footer { padding: 10px; }
  #sidebar-footer button { font-size: 11px; padding: 7px; }
}