/* ===== CSS Variables ===== */
:root {
  --color-bg: #f0f2f5;
  --color-surface: #ffffff;
  --color-primary: #4f46e5;
  --color-primary-hover: #4338ca;
  --color-primary-light: #ede9fe;
  --color-danger: #dc2626;
  --color-danger-hover: #b91c1c;
  --color-danger-light: #fee2e2;
  --color-success: #16a34a;
  --color-success-hover: #15803d;
  --color-success-light: #dcfce7;
  --color-text: #111827;
  --color-text-muted: #6b7280;
  --color-border: #e5e7eb;
  --color-card-front: #4f46e5;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow: 0 1px 3px rgba(0,0,0,.08), 0 4px 12px rgba(0,0,0,.06);
  --shadow-lg: 0 8px 32px rgba(0,0,0,.12);
  --transition: .2s ease;
}

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

html { font-size: 16px; }

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  line-height: 1.5;
  min-height: 100vh;
}

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

/* ===== Layout ===== */
.container {
  max-width: 720px;
  margin: 0 auto;
  padding: 0 16px 48px;
}

/* ===== Navigation ===== */
.nav {
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  position: sticky;
  top: 0;
  z-index: 100;
  box-shadow: 0 1px 4px rgba(0,0,0,.06);
}

.nav-inner {
  max-width: 720px;
  margin: 0 auto;
  padding: 0 16px;
  height: 56px;
  display: flex;
  align-items: center;
  gap: 12px;
}

.nav-brand {
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--color-primary);
  display: flex;
  align-items: center;
  gap: 6px;
}

.nav-brand .icon { font-size: 1.3rem; }

.nav-back {
  display: flex;
  align-items: center;
  gap: 4px;
  color: var(--color-text-muted);
  font-size: .9rem;
  font-weight: 500;
  transition: color var(--transition);
  padding: 4px 8px;
  border-radius: var(--radius-sm);
}

.nav-back:hover { color: var(--color-primary); background: var(--color-primary-light); }

.nav-title {
  flex: 1;
  font-weight: 600;
  font-size: 1rem;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.nav-counter {
  font-size: .85rem;
  color: var(--color-text-muted);
  font-weight: 500;
  white-space: nowrap;
}

/* ===== Page Header ===== */
.page-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 28px 0 20px;
  gap: 12px;
}

.page-header h1 {
  font-size: 1.5rem;
  font-weight: 700;
}

/* ===== Buttons ===== */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  padding: 10px 18px;
  border-radius: var(--radius-sm);
  border: none;
  font-size: .9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background var(--transition), transform .1s, box-shadow var(--transition);
  min-height: 40px;
  touch-action: manipulation;
  white-space: nowrap;
}

.btn:active { transform: scale(.97); }

.btn-primary {
  background: var(--color-primary);
  color: #fff;
}
.btn-primary:hover { background: var(--color-primary-hover); }

.btn-danger {
  background: var(--color-danger);
  color: #fff;
}
.btn-danger:hover { background: var(--color-danger-hover); }

.btn-success {
  background: var(--color-success);
  color: #fff;
}
.btn-success:hover { background: var(--color-success-hover); }

.btn-ghost {
  background: transparent;
  color: var(--color-text-muted);
  border: 1px solid var(--color-border);
}
.btn-ghost:hover { background: var(--color-bg); color: var(--color-text); }

.btn-outline-danger {
  background: transparent;
  color: var(--color-danger);
  border: 1px solid var(--color-danger);
}
.btn-outline-danger:hover { background: var(--color-danger-light); }

.btn-sm {
  padding: 6px 12px;
  font-size: .8rem;
  min-height: 32px;
}

.btn-lg {
  padding: 14px 28px;
  font-size: 1rem;
  min-height: 52px;
  border-radius: var(--radius);
}

/* ===== Set Cards (Home) ===== */
.sets-list { display: flex; flex-direction: column; gap: 12px; }

.set-card {
  background: var(--color-surface);
  border-radius: var(--radius);
  padding: 20px;
  box-shadow: var(--shadow);
  display: flex;
  flex-direction: column;
  gap: 12px;
  transition: box-shadow var(--transition), transform var(--transition);
}

.set-card:hover { box-shadow: var(--shadow-lg); transform: translateY(-1px); }

.set-card-header { display: flex; align-items: flex-start; justify-content: space-between; gap: 8px; }

.set-card-title {
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--color-text);
}

.set-card-meta {
  font-size: .8rem;
  color: var(--color-text-muted);
  margin-top: 2px;
}

.set-card-badges { display: flex; gap: 6px; flex-wrap: wrap; align-items: center; }

.badge {
  display: inline-flex;
  align-items: center;
  padding: 3px 9px;
  border-radius: 99px;
  font-size: .75rem;
  font-weight: 600;
}

.badge-due { background: #fef3c7; color: #92400e; }
.badge-done { background: var(--color-success-light); color: #166534; }

/* Progress bar */
.progress-wrap { display: flex; flex-direction: column; gap: 4px; }
.progress-label { font-size: .78rem; color: var(--color-text-muted); }
.progress-bar-bg {
  height: 6px;
  background: var(--color-border);
  border-radius: 99px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  background: var(--color-success);
  border-radius: 99px;
  transition: width .5s ease;
}

.set-card-actions {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
}

/* ===== Empty State ===== */
.empty-state {
  text-align: center;
  padding: 64px 24px;
  color: var(--color-text-muted);
}

.empty-state .icon-big { font-size: 4rem; margin-bottom: 16px; opacity: .5; }
.empty-state h2 { font-size: 1.2rem; color: var(--color-text); margin-bottom: 8px; }
.empty-state p { font-size: .95rem; margin-bottom: 24px; }

/* ===== Forms ===== */
.form-group { display: flex; flex-direction: column; gap: 6px; margin-bottom: 20px; }

label {
  font-size: .85rem;
  font-weight: 600;
  color: var(--color-text);
}

input[type="text"],
textarea {
  width: 100%;
  padding: 10px 14px;
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-sm);
  font-size: 1rem;
  font-family: inherit;
  color: var(--color-text);
  background: var(--color-surface);
  transition: border-color var(--transition), box-shadow var(--transition);
  resize: vertical;
}

input[type="text"]:focus,
textarea:focus {
  outline: none;
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(79, 70, 229, .15);
}

textarea { font-family: 'Courier New', monospace; font-size: .88rem; min-height: 280px; }

.json-toolbar {
  display: flex;
  gap: 8px;
  margin-bottom: 8px;
  flex-wrap: wrap;
}

.json-feedback {
  margin-top: 8px;
  font-size: .85rem;
  padding: 8px 12px;
  border-radius: var(--radius-sm);
  display: none;
}

.json-feedback.valid { display: block; background: var(--color-success-light); color: #166534; }
.json-feedback.invalid { display: block; background: var(--color-danger-light); color: #991b1b; }

.form-actions {
  display: flex;
  gap: 10px;
  justify-content: flex-end;
  margin-top: 8px;
  flex-wrap: wrap;
}

/* ===== Modal ===== */
.modal-backdrop {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,.45);
  z-index: 200;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 16px;
  animation: fadeIn .15s ease;
}

.modal-backdrop[hidden] { display: none; }

.modal-box {
  background: var(--color-surface);
  border-radius: var(--radius);
  padding: 28px;
  max-width: 400px;
  width: 100%;
  box-shadow: var(--shadow-lg);
  animation: slideUp .18s ease;
}

.modal-box h2 { font-size: 1.1rem; margin-bottom: 10px; }
.modal-box p { color: var(--color-text-muted); font-size: .9rem; margin-bottom: 24px; }
.modal-actions { display: flex; gap: 10px; justify-content: flex-end; }

@keyframes fadeIn { from { opacity: 0; } to { opacity: 1; } }
@keyframes slideUp { from { transform: translateY(12px); opacity: 0; } to { transform: translateY(0); opacity: 1; } }

/* ===== Toast ===== */
.toast {
  position: fixed;
  bottom: 24px;
  right: 24px;
  background: #1f2937;
  color: #fff;
  padding: 12px 20px;
  border-radius: var(--radius);
  font-size: .9rem;
  font-weight: 500;
  box-shadow: var(--shadow-lg);
  z-index: 300;
  animation: slideInRight .2s ease;
  max-width: 320px;
}

.toast[hidden] { display: none; }

@keyframes slideInRight {
  from { transform: translateX(20px); opacity: 0; }
  to { transform: translateX(0); opacity: 1; }
}

/* ===== Topic textarea (step 1) ===== */
.topic-textarea {
  font-family: inherit;
  font-size: 1rem;
  min-height: 140px;
  resize: vertical;
  line-height: 1.6;
}

/* ===== Swap toggle ===== */
.swap-toggle-wrap {
  width: 100%;
  display: flex;
  justify-content: flex-end;
  margin-bottom: 12px;
}

.swap-toggle {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  background: transparent;
  border: 1.5px solid var(--color-border);
  border-radius: 99px;
  padding: 6px 14px;
  font-size: .82rem;
  font-weight: 600;
  color: var(--color-text-muted);
  cursor: pointer;
  transition: background var(--transition), color var(--transition), border-color var(--transition);
  touch-action: manipulation;
}

.swap-toggle:hover {
  background: var(--color-primary-light);
  color: var(--color-primary);
  border-color: var(--color-primary);
}

.swap-toggle.is-active {
  background: var(--color-primary);
  color: #fff;
  border-color: var(--color-primary);
}

.swap-icon { font-size: 1rem; }

/* ===== Wizard / Instruction Card ===== */
.instruction-card {
  background: var(--color-surface);
  border-radius: var(--radius);
  padding: 24px;
  box-shadow: var(--shadow);
  display: flex;
  flex-direction: column;
  gap: 16px;
}

.instruction-step {
  display: flex;
  align-items: flex-start;
  gap: 14px;
  font-size: .95rem;
  line-height: 1.5;
}

.step-num {
  flex-shrink: 0;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--color-primary);
  color: #fff;
  font-size: .82rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  justify-content: center;
  margin-top: 1px;
}

/* ===== Study Page ===== */
.study-container {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 24px 16px 48px;
  min-height: calc(100vh - 56px);
}

/* Session progress bar (top) */
.session-progress-bar {
  width: 100%;
  max-width: 720px;
  height: 8px;
  background: var(--color-border);
  border-radius: 99px;
  margin-bottom: 32px;
  overflow: hidden;
  display: flex;
  gap: 2px;
}

.session-progress-bar.has-segments {
  background: transparent;
}

.progress-seg {
  flex: 1;
  height: 100%;
  border-radius: 99px;
  animation: segPop .2s ease;
}

.progress-seg.known     { background: var(--color-success); }
.progress-seg.unknown   { background: var(--color-danger); }
.progress-seg.remaining { background: var(--color-border); }

@keyframes segPop {
  from { transform: scaleY(0); opacity: 0; }
  to   { transform: scaleY(1); opacity: 1; }
}

/* 3D Flip Card */
.card-scene {
  width: 100%;
  max-width: 480px;
  perspective: 1200px;
  margin-bottom: 28px;
}

.flip-card {
  width: 100%;
  min-height: 260px;
  position: relative;
  transform-style: preserve-3d;
  transition: transform .5s cubic-bezier(.4,0,.2,1);
  cursor: pointer;
  touch-action: manipulation;
  user-select: none;
}

.flip-card.is-flipped {
  transform: rotateY(180deg);
  cursor: default;
}

.card-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  border-radius: var(--radius);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 32px 24px;
  box-shadow: var(--shadow-lg);
}

.card-front {
  background: var(--color-card-front);
  color: #fff;
}

.card-back {
  background: var(--color-surface);
  color: var(--color-text);
  transform: rotateY(180deg);
}

.card-word {
  font-size: 2rem;
  font-weight: 700;
  text-align: center;
  word-break: break-word;
  line-height: 1.2;
  margin-bottom: 8px;
}

.card-hint {
  font-size: .9rem;
  opacity: .75;
  text-align: center;
  font-style: italic;
}

.card-flip-prompt {
  margin-top: 20px;
  font-size: .8rem;
  opacity: .6;
  letter-spacing: .04em;
  text-transform: uppercase;
}

.card-translation {
  font-size: 1.6rem;
  font-weight: 700;
  text-align: center;
  color: var(--color-primary);
  margin-bottom: 8px;
}

.card-original {
  font-size: 1rem;
  color: var(--color-text-muted);
  text-align: center;
  margin-bottom: 24px;
}

/* Response buttons */
.response-buttons {
  display: flex;
  gap: 12px;
  width: 100%;
  margin-top: 8px;
}

.response-buttons .btn { flex: 1; }

/* Session complete */
.session-complete {
  text-align: center;
  padding: 32px 16px;
  max-width: 400px;
  width: 100%;
}

.session-complete h2 { font-size: 1.8rem; margin-bottom: 8px; }
.session-complete .subtitle { color: var(--color-text-muted); margin-bottom: 32px; }

.stats-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 12px;
  margin-bottom: 32px;
}

.stat-box {
  background: var(--color-surface);
  border-radius: var(--radius);
  padding: 20px 12px;
  box-shadow: var(--shadow);
}

.stat-box .stat-num { font-size: 2rem; font-weight: 800; color: var(--color-primary); }
.stat-box .stat-label { font-size: .8rem; color: var(--color-text-muted); margin-top: 2px; }

.session-complete-actions { display: flex; flex-direction: column; gap: 10px; align-items: center; }

/* All caught up */
.caught-up {
  text-align: center;
  padding: 48px 24px;
}

.caught-up .icon-big { font-size: 3rem; margin-bottom: 12px; }
.caught-up h2 { font-size: 1.3rem; margin-bottom: 8px; }
.caught-up p { color: var(--color-text-muted); margin-bottom: 24px; }

/* ===== Responsive ===== */
@media (min-width: 640px) {
  .page-header h1 { font-size: 1.75rem; }
  .set-card { flex-direction: row; align-items: center; flex-wrap: wrap; }
  .set-card-info { flex: 1; min-width: 0; }
  .set-card-actions { flex-shrink: 0; }
  .response-buttons .btn { min-height: 56px; font-size: 1.05rem; }
  .card-word { font-size: 2.4rem; }
  .card-translation { font-size: 1.9rem; }
}