/* ── CSS Variables ─────────────────────────────────────────── */
:root {
  --sidebar-width: 290px;
  --header-height: 52px;
  --transition: 0.28s cubic-bezier(0.4, 0, 0.2, 1);
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 14px;
}

body.dark {
  --bg-page:       #0f1117;
  --bg-sidebar:    #161820;
  --bg-surface:    #1c1f2a;
  --bg-card:       #232636;
  --bg-card-back:  #1a2035;
  --bg-hover:      rgba(255,255,255,0.06);
  --bg-active:     rgba(99,102,241,0.18);
  --bg-input:      #2a2d3e;
  --bg-btn:        #2a2d3e;
  --bg-btn-hover:  #333650;
  --bg-correct:    rgba(34,197,94,0.15);
  --bg-wrong:      rgba(239,68,68,0.15);
  --bg-reveal:     rgba(59,130,246,0.12);
  --color-text:    #e2e4ee;
  --color-muted:   #8b8fa8;
  --color-dim:     #5a5e75;
  --color-accent:  #818cf8;
  --color-accent2: #6366f1;
  --color-border:  rgba(255,255,255,0.08);
  --color-border2: rgba(255,255,255,0.14);
  --color-correct: #4ade80;
  --color-wrong:   #f87171;
  --color-reveal:  #60a5fa;
  --shadow-card:   0 4px 24px rgba(0,0,0,0.45);
  --shadow-flip:   0 12px 40px rgba(0,0,0,0.6);
}

body.light {
  --bg-page:       #f5f5f7;
  --bg-sidebar:    #ffffff;
  --bg-surface:    #ffffff;
  --bg-card:       #ffffff;
  --bg-card-back:  #f0f4ff;
  --bg-hover:      rgba(0,0,0,0.05);
  --bg-active:     rgba(99,102,241,0.1);
  --bg-input:      #f3f4f6;
  --bg-btn:        #f3f4f6;
  --bg-btn-hover:  #e5e7eb;
  --bg-correct:    rgba(34,197,94,0.12);
  --bg-wrong:      rgba(239,68,68,0.1);
  --bg-reveal:     rgba(59,130,246,0.08);
  --color-text:    #111827;
  --color-muted:   #6b7280;
  --color-dim:     #9ca3af;
  --color-accent:  #6366f1;
  --color-accent2: #4f46e5;
  --color-border:  rgba(0,0,0,0.08);
  --color-border2: rgba(0,0,0,0.14);
  --color-correct: #16a34a;
  --color-wrong:   #dc2626;
  --color-reveal:  #2563eb;
  --shadow-card:   0 2px 12px rgba(0,0,0,0.08), 0 1px 3px rgba(0,0,0,0.06);
  --shadow-flip:   0 8px 32px rgba(0,0,0,0.15);
}

/* ── Reset & Base ──────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html, body { height: 100%; overflow: hidden; }
body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  background: var(--bg-page);
  color: var(--color-text);
  font-size: 15px;
  line-height: 1.5;
  transition: background var(--transition), color var(--transition);
}
button { cursor: pointer; border: none; background: none; font: inherit; color: inherit; }
input { font: inherit; color: inherit; }

/* ── Layout ────────────────────────────────────────────────── */
#app { display: flex; height: 100vh; overflow: hidden; }

/* ── Header ────────────────────────────────────────────────── */
#header {
  position: fixed; top: 0; left: 0; right: 0;
  height: var(--header-height);
  background: var(--bg-sidebar);
  border-bottom: 1px solid var(--color-border);
  display: flex; align-items: center;
  padding: 0 16px;
  z-index: 100;
  gap: 12px;
  transition: background var(--transition);
}
#header-title {
  font-size: 16px; font-weight: 600;
  color: var(--color-accent);
  letter-spacing: -0.3px;
  flex: 1;
}
#btn-sidebar-toggle, #btn-theme {
  width: 34px; height: 34px;
  display: flex; align-items: center; justify-content: center;
  border-radius: var(--radius-sm);
  background: var(--bg-btn);
  transition: background 0.15s;
  font-size: 16px;
  color: var(--color-muted);
  flex-shrink: 0;
}
#btn-sidebar-toggle:hover, #btn-theme:hover { background: var(--bg-btn-hover); color: var(--color-text); }

/* ── Sidebar ────────────────────────────────────────────────── */
#sidebar {
  position: fixed; top: var(--header-height); left: 0; bottom: 0;
  width: var(--sidebar-width);
  background: var(--bg-sidebar);
  border-right: 1px solid var(--color-border);
  overflow-y: auto;
  overflow-x: hidden;
  transition: transform var(--transition), background var(--transition);
  z-index: 50;
  padding: 8px 0 24px;
}
#sidebar.hidden { transform: translateX(calc(-1 * var(--sidebar-width))); }

#sidebar::-webkit-scrollbar { width: 4px; }
#sidebar::-webkit-scrollbar-track { background: transparent; }
#sidebar::-webkit-scrollbar-thumb { background: var(--color-border2); border-radius: 4px; }

/* ── Sidebar: themes & files ────────────────────────────────── */
.theme-group { margin-bottom: 2px; }
.theme-header {
  display: flex; align-items: center;
  padding: 7px 14px 7px 12px;
  gap: 8px;
  cursor: pointer;
  border-radius: 0;
  transition: background 0.12s;
  user-select: none;
}
.theme-header:hover { background: var(--bg-hover); }
.theme-icon {
  font-size: 13px;
  color: var(--color-dim);
  transition: transform 0.2s;
  flex-shrink: 0;
  display: inline-block;
}
.theme-icon.open { transform: rotate(90deg); }
.theme-label { font-size: 11px; font-weight: 600; text-transform: uppercase; letter-spacing: 0.8px; color: var(--color-muted); }
.theme-files { overflow: hidden; transition: max-height 0.25s ease; }

.file-item {
  display: flex; flex-direction: column;
  padding: 7px 10px 7px 16px;
  cursor: pointer;
  border-radius: var(--radius-sm);
  margin: 0 6px;
  transition: background 0.12s;
}
.file-item:hover { background: var(--bg-hover); }
.file-item.active { background: var(--bg-active); }
.file-item.active .file-name { color: var(--color-accent); font-weight: 500; }
.file-item-top { display: flex; align-items: center; gap: 6px; margin-bottom: 5px; }
.file-name { font-size: 13px; color: var(--color-text); flex: 1; min-width: 0; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.file-score { font-size: 10px; color: var(--color-muted); font-weight: 500; white-space: nowrap; flex-shrink: 0; }
.file-restart {
  opacity: 0; font-size: 11px; color: var(--color-dim);
  padding: 1px 4px; border-radius: var(--radius-sm);
  transition: opacity 0.15s, background 0.12s, color 0.12s;
  flex-shrink: 0; line-height: 1.4;
}
.file-item:hover .file-restart { opacity: 1; }
.file-restart:hover { background: var(--bg-btn-hover); color: var(--color-text); }
.file-progress-wrap {
  height: 3px;
  background: var(--color-border);
  border-radius: 2px;
  overflow: hidden;
}
.file-progress-bar {
  height: 100%; width: 0%;
  background: var(--color-accent2);
  border-radius: 2px;
  transition: width 0.35s ease;
}
.file-item.active .file-progress-bar { background: var(--color-accent); }

/* ── Main content area ──────────────────────────────────────── */
#main {
  margin-top: var(--header-height);
  margin-left: var(--sidebar-width);
  flex: 1;
  transition: margin-left var(--transition);
  overflow-y: auto;
  height: calc(100vh - var(--header-height));
  background: var(--bg-page);
  transition: background var(--transition), margin-left var(--transition);
}
#main.full { margin-left: 0; }

#main::-webkit-scrollbar { width: 6px; }
#main::-webkit-scrollbar-track { background: transparent; }
#main::-webkit-scrollbar-thumb { background: var(--color-border2); border-radius: 4px; }

/* ── Welcome / empty state ─────────────────────────────────── */
#welcome {
  display: flex; flex-direction: column; align-items: center; justify-content: center;
  height: 100%; gap: 12px;
  padding: 40px;
  text-align: center;
}
.welcome-icon { font-size: 48px; margin-bottom: 8px; }
.welcome-title { font-size: 22px; font-weight: 600; color: var(--color-text); }
.welcome-sub { font-size: 14px; color: var(--color-muted); max-width: 320px; line-height: 1.6; }

/* ── Content panel ──────────────────────────────────────────── */
#content-panel { display: none; flex-direction: column; height: 100%; }
#content-panel.visible { display: flex; }

/* ── Tab bar ────────────────────────────────────────────────── */
#tab-bar {
  display: flex; align-items: center;
  padding: 0 24px;
  border-bottom: 1px solid var(--color-border);
  background: var(--bg-surface);
  flex-shrink: 0;
  gap: 4px;
  height: 48px;
  transition: background var(--transition);
}
.tab-btn {
  padding: 6px 16px;
  font-size: 14px; font-weight: 500;
  border-radius: var(--radius-sm);
  color: var(--color-muted);
  transition: background 0.12s, color 0.12s;
}
.tab-btn:hover { background: var(--bg-hover); color: var(--color-text); }
.tab-btn.active { background: var(--bg-active); color: var(--color-accent); }

.tab-spacer { flex: 1; }
.shuffle-wrap { display: flex; align-items: center; gap: 8px; }
.shuffle-label { font-size: 12px; color: var(--color-muted); }
.toggle-btn {
  width: 38px; height: 22px;
  border-radius: 11px;
  background: var(--color-dim);
  position: relative; cursor: pointer;
  transition: background 0.2s;
  flex-shrink: 0;
  border: none;
}
.toggle-btn.on { background: var(--color-accent2); }
.toggle-btn::after {
  content: '';
  position: absolute; top: 3px; left: 3px;
  width: 16px; height: 16px;
  border-radius: 50%; background: #fff;
  transition: transform 0.2s;
}
.toggle-btn.on::after { transform: translateX(16px); }

/* ── Tab sections ───────────────────────────────────────────── */
#tab-flashcards, #tab-quiz, #tab-results { display: none; flex: 1; overflow-y: auto; }
#tab-flashcards.active, #tab-quiz.active, #tab-results.active { display: flex; flex-direction: column; }

/* ── Flashcard scene ────────────────────────────────────────── */
.flashcard-scene {
  display: flex; flex-direction: column; align-items: center;
  padding: 32px 24px 24px;
  flex: 1;
}
.progress-text { font-size: 13px; color: var(--color-muted); margin-bottom: 24px; font-weight: 500; }

.card-3d-wrap {
  perspective: 1000px;
  width: 100%; max-width: 600px; min-height: 260px;
  cursor: pointer;
  margin-bottom: 32px;
}
.card-3d {
  width: 100%; height: 100%; min-height: 260px;
  position: relative;
  transform-style: preserve-3d;
  transition: transform 0.55s cubic-bezier(0.4, 0, 0.2, 1);
}
.card-3d.flipped { transform: rotateY(180deg); }

.card-face {
  position: absolute; inset: 0;
  border-radius: var(--radius-lg);
  padding: 32px;
  display: flex; flex-direction: column; align-items: center; justify-content: center;
  text-align: center;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  box-shadow: var(--shadow-card);
  transition: background var(--transition);
}
.card-face-front {
  background: var(--bg-card);
  border: 1px solid var(--color-border);
}
.card-face-back {
  background: var(--bg-card-back);
  border: 1px solid var(--color-border);
  transform: rotateY(180deg);
}

.card-face-label {
  font-size: 10px; font-weight: 600; text-transform: uppercase; letter-spacing: 1px;
  color: var(--color-dim); margin-bottom: 16px;
}
.card-face-text { font-size: 17px; line-height: 1.6; color: var(--color-text); }
.card-flip-hint {
  font-size: 12px; color: var(--color-dim); margin-top: 16px;
  display: flex; align-items: center; gap: 6px;
}

.card-controls {
  display: flex; align-items: center; gap: 24px;
}
.nav-btn {
  width: 44px; height: 44px;
  display: flex; align-items: center; justify-content: center;
  border-radius: 50%;
  background: var(--bg-btn);
  font-size: 18px; color: var(--color-muted);
  transition: background 0.12s, color 0.12s, transform 0.1s;
}
.nav-btn:hover:not(:disabled) { background: var(--bg-btn-hover); color: var(--color-text); }
.nav-btn:active:not(:disabled) { transform: scale(0.93); }
.nav-btn:disabled { opacity: 0.35; cursor: not-allowed; }

/* ── Quiz ───────────────────────────────────────────────────── */
#tab-quiz { display: none; flex-direction: column; flex: 1; overflow: hidden; }
#tab-quiz.active { display: flex; }
.quiz-nav-bar {
  display: flex; align-items: center; gap: 12px;
  padding: 10px 24px;
  border-bottom: 1px solid var(--color-border);
  background: var(--bg-surface);
  flex-shrink: 0;
  transition: background var(--transition);
}
.progress-text-quiz { flex: 1; text-align: center; font-size: 13px; color: var(--color-muted); font-weight: 500; }
.quiz-scene {
  display: flex; flex-direction: column; align-items: center;
  padding: 32px 24px 24px;
  flex: 1;
  overflow-y: auto;
}
.quiz-card {
  width: 100%; max-width: 620px;
  background: var(--bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 28px 28px 24px;
  box-shadow: var(--shadow-card);
  transition: background var(--transition);
}
.quiz-question { font-size: 17px; line-height: 1.6; color: var(--color-text); margin-bottom: 22px; font-weight: 500; }

.options-list { display: flex; flex-direction: column; gap: 10px; }
.option-btn {
  width: 100%; padding: 12px 16px;
  text-align: left; font-size: 15px;
  border: 1.5px solid var(--color-border2);
  border-radius: var(--radius-md);
  background: var(--bg-input);
  color: var(--color-text);
  transition: background 0.12s, border-color 0.12s, transform 0.08s;
  line-height: 1.4;
}
.option-btn:hover:not(:disabled) { background: var(--bg-btn-hover); border-color: var(--color-accent); }
.option-btn:active:not(:disabled) { transform: scale(0.99); }
.option-btn:disabled { cursor: default; }
.option-btn.correct { background: var(--bg-correct); border-color: var(--color-correct); color: var(--color-correct); }
.option-btn.wrong { background: var(--bg-wrong); border-color: var(--color-wrong); color: var(--color-wrong); }
.option-btn.reveal { background: var(--bg-reveal); border-color: var(--color-reveal); color: var(--color-reveal); }

.tf-row { display: flex; gap: 12px; }
.tf-btn {
  flex: 1; padding: 12px 16px;
  font-size: 15px; font-weight: 500;
  border: 1.5px solid var(--color-border2);
  border-radius: var(--radius-md);
  background: var(--bg-input);
  color: var(--color-text);
  transition: background 0.12s, border-color 0.12s;
}
.tf-btn:hover:not(:disabled) { background: var(--bg-btn-hover); border-color: var(--color-accent); }
.tf-btn:disabled { cursor: default; }
.tf-btn.correct { background: var(--bg-correct); border-color: var(--color-correct); color: var(--color-correct); }
.tf-btn.wrong { background: var(--bg-wrong); border-color: var(--color-wrong); color: var(--color-wrong); }

.text-answer-wrap { display: flex; flex-direction: column; gap: 10px; }
.text-area { resize: vertical; min-height: 90px; line-height: 1.5; }
.text-input {
  flex: 1; padding: 10px 14px;
  border: 1.5px solid var(--color-border2);
  border-radius: var(--radius-md);
  background: var(--bg-input);
  color: var(--color-text);
  font-size: 15px;
  outline: none;
  transition: border-color 0.15s;
}
.text-input:focus { border-color: var(--color-accent); }
.text-input:disabled { opacity: 0.7; cursor: default; }
.submit-btn {
  padding: 10px 18px;
  border: 1.5px solid var(--color-accent2);
  border-radius: var(--radius-md);
  background: var(--color-accent2);
  color: #fff;
  font-size: 15px; font-weight: 500;
  transition: opacity 0.15s, transform 0.08s;
  white-space: nowrap;
  flex-shrink: 0;
}
.submit-btn:hover { opacity: 0.88; }
.submit-btn:active { transform: scale(0.97); }
.submit-btn:disabled { opacity: 0.45; cursor: default; }

.feedback-row {
  margin-top: 14px; font-size: 14px; font-weight: 500;
  display: flex; align-items: center; gap: 8px;
}
.feedback-row.correct { color: var(--color-correct); }
.feedback-row.wrong { color: var(--color-wrong); }
.feedback-correct-reveal { color: var(--color-reveal); font-weight: 400; }
.feedback-row.neutral { color: var(--color-muted); }
.feedback-ref-answer { color: var(--color-correct); font-weight: 400; display: block; margin-top: 6px; }
.feedback-row.neutral { flex-direction: column; align-items: flex-start; }



.finish-btn {
  padding: 10px 20px;
  border: 1.5px solid var(--color-accent2);
  border-radius: var(--radius-md);
  background: var(--color-accent2);
  color: #fff; font-size: 14px; font-weight: 500;
  transition: opacity 0.15s, transform 0.08s;
}
.finish-btn:hover { opacity: 0.88; }
.finish-btn:active { transform: scale(0.97); }

/* ── Results ─────────────────────────────────────────────────── */
.results-scene {
  display: flex; flex-direction: column; align-items: center;
  padding: 40px 24px 40px;
  flex: 1;
  gap: 28px;
}
.score-card {
  background: var(--bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 32px 40px;
  text-align: center;
  box-shadow: var(--shadow-card);
  min-width: 280px;
}
.score-num { font-size: 44px; font-weight: 700; line-height: 1; margin-bottom: 6px; }
.score-pct { font-size: 20px; font-weight: 600; color: var(--color-muted); }
.score-label { font-size: 14px; color: var(--color-dim); margin-top: 8px; }

.score-card.great .score-num { color: var(--color-correct); }
.score-card.ok .score-num { color: #f59e0b; }
.score-card.low .score-num { color: var(--color-wrong); }

.results-actions { display: flex; gap: 12px; flex-wrap: wrap; justify-content: center; }
.results-actions button {
  padding: 10px 22px;
  border-radius: var(--radius-md);
  font-size: 14px; font-weight: 500;
  transition: opacity 0.15s, transform 0.08s;
}
.results-actions button:active { transform: scale(0.97); }
.btn-retake {
  background: var(--color-accent2); color: #fff;
  border: 1.5px solid var(--color-accent2);
}
.btn-back-study {
  background: var(--bg-btn); color: var(--color-text);
  border: 1.5px solid var(--color-border2);
}
.btn-retake:hover { opacity: 0.88; }
.btn-back-study:hover { background: var(--bg-btn-hover); }

.review-list { width: 100%; max-width: 640px; display: flex; flex-direction: column; gap: 12px; }
.review-item {
  background: var(--bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 14px 16px;
  font-size: 14px;
  transition: background var(--transition);
}
.review-item.correct-item { border-left: 3px solid var(--color-correct); }
.review-item.open-item { border-left: 3px solid var(--color-muted); }
.review-item.wrong-item { border-left: 3px solid var(--color-wrong); }
.review-q { font-weight: 500; color: var(--color-text); margin-bottom: 8px; display: flex; gap: 8px; align-items: flex-start; }
.review-indicator { flex-shrink: 0; font-size: 15px; margin-top: 1px; }
.review-indicator.ok { color: var(--color-correct); }
.review-indicator.open { color: var(--color-muted); }
.review-indicator.bad { color: var(--color-wrong); }
.review-answers { display: flex; flex-direction: column; gap: 4px; padding-left: 24px; }
.review-ans-row { font-size: 13px; color: var(--color-muted); }
.review-ans-row span { font-weight: 500; }
.review-ans-row.user-correct span { color: var(--color-correct); }
.review-ans-row.user-wrong span { color: var(--color-wrong); }
.review-ans-row.correct-ans span { color: var(--color-correct); }
.review-ans-row.user-open span { color: var(--color-text); }
.review-ans-row.ref-ans { font-style: italic; }
.review-ans-row.ref-ans span { color: var(--color-muted); }

/* ── Loading ─────────────────────────────────────────────────── */
#loading-overlay {
  display: none; position: fixed; inset: 0;
  background: rgba(0,0,0,0.4);
  align-items: center; justify-content: center;
  z-index: 200;
}
#loading-overlay.visible { display: flex; }
.spinner {
  width: 36px; height: 36px;
  border: 3px solid var(--color-border2);
  border-top-color: var(--color-accent);
  border-radius: 50%;
  animation: spin 0.7s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }

/* ── Responsive ──────────────────────────────────────────────── */
@media (max-width: 640px) {
  :root { --sidebar-width: 240px; }
  .card-face { padding: 24px 20px; }
  .card-face-text { font-size: 15px; }
  .quiz-card { padding: 20px; }
  .quiz-question { font-size: 15px; }
}