:root{
  --primary: #ff6b35;
  --primary-dark: #e8531f;
  --secondary: #ff9142;
  --danger: #e63946;
  --success: #2a9d5c;
  --success-bg: #e8f8ef;
  --danger-bg: #fdecee;
  --bg: #fff8f4;
  --card-bg: #ffffff;
  --text-main: #2b2320;
  --text-muted: #8a7a70;
  --border: #f0e0d6;
  --radius-lg: 22px;
  --radius-md: 16px;
  --radius-sm: 10px;
  --shadow: 0 4px 16px rgba(230, 90, 40, 0.08);
  --shadow-hover: 0 8px 24px rgba(230, 90, 40, 0.16);
  --tint-bg: #fff3ec;

  /* Màu phụ từng module (Dashboard) — thương hiệu vẫn cam/đỏ làm chủ đạo */
  --mod-listening: #ff6b35;
  --mod-reading: #2a6fe8;
  --mod-cloze: #8b5cf6;
  --mod-shortreading: #2a9d5c;
  --mod-material: #e0a52c;
  --mod-vocabulary: #e0568f;
}

[data-theme="dark"]{
  --primary: #ff8154;
  --primary-dark: #ff6b35;
  --secondary: #ffa564;
  --danger: #ff6b7a;
  --success: #4ecb86;
  --success-bg: #143324;
  --danger-bg: #3a1d22;
  --bg: #1f1815;
  --card-bg: #2a211e;
  --text-main: #f5ece6;
  --text-muted: #b8a89e;
  --border: #46372f;
  --shadow: 0 4px 16px rgba(0, 0, 0, 0.3);
  --shadow-hover: 0 8px 28px rgba(0, 0, 0, 0.4);
  --tint-bg: #382720;
}

*{ box-sizing: border-box; }

body{
  margin: 0;
  background: var(--bg);
  color: var(--text-main);
  font-family: 'Be Vietnam Pro', 'Noto Sans TC', -apple-system, sans-serif;
  line-height: 1.6;
  transition: background 0.2s ease, color 0.2s ease;
}

/* Font tiếng Trung: ưu tiên font hệ thống DFKai-SB/BiauKai/KaiTi (Windows/Mac
   có cài tiếng Trung phồn thể) — đây là font hệ thống, không nhúng web được.
   Máy không có sẽ tự dùng cwTeX Q Kai (web font) làm phương án dự phòng. */
.zh{ font-family: 'DFKai-SB', 'BiauKai', 'KaiTi', 'cwTeX Q Kai', 'Noto Serif TC', serif; }

.container{
  max-width: 960px;
  margin: 0 auto;
  padding: 0 20px;
}

.hidden{ display: none !important; }

/* ===== HIỆU ỨNG CHUYỂN TRANG ===== */
@keyframes fadeSlideIn{
  from{ opacity: 0; transform: translateY(10px); }
  to{ opacity: 1; transform: translateY(0); }
}

#app-root{ animation: fadeSlideIn 0.22s ease; }

/* ===== HEADER ===== */
.site-header{
  background: linear-gradient(135deg, var(--primary), var(--secondary));
  padding: 20px 0;
  color: #fff;
  box-shadow: 0 2px 12px rgba(230, 90, 40, 0.25);
}

.header-inner{
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 10px;
}

.header-actions{
  display: flex;
  align-items: center;
  gap: 10px;
}

.theme-toggle-btn{
  width: 38px;
  height: 38px;
  border-radius: 50%;
  border: none;
  background: rgba(255,255,255,0.22);
  color: #fff;
  font-size: 18px;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
}

.theme-toggle-btn:hover{ background: rgba(255,255,255,0.32); }

.site-title{
  margin: 0;
  font-size: 26px;
  font-weight: 900;
  letter-spacing: 0.3px;
}

.site-subtitle{
  margin: 4px 0 0;
  font-size: 13px;
  opacity: 0.95;
}

.student-badge{
  font-size: 13px;
  background: rgba(255,255,255,0.18);
  padding: 8px 14px;
  border-radius: 20px;
  white-space: nowrap;
}

.link-btn{
  background: none;
  border: none;
  color: #fff;
  text-decoration: underline;
  cursor: pointer;
  font-size: 13px;
  font-family: inherit;
  padding: 0 0 0 6px;
}

.site-footer{
  text-align: center;
  padding: 24px 16px 40px;
  color: var(--text-muted);
  font-size: 13px;
}

main{ padding: 28px 0 64px; min-height: 60vh; }

/* ===== DASHBOARD ===== */
.dashboard-greeting{ margin: 0 0 4px; font-size: 26px; font-weight: 900; }
.dashboard-subgreeting{ color: var(--text-muted); margin: 0 0 28px; font-size: 15px; }

.module-grid{
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 18px;
  margin-bottom: 32px;
}

.module-tile{
  display: block;
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  padding: 24px 20px;
  box-shadow: var(--shadow);
  border: 1px solid var(--border);
  text-decoration: none;
  color: var(--text-main);
  border-top: 4px solid var(--tile-color, var(--primary));
  transition: box-shadow 0.15s ease, transform 0.15s ease;
  position: relative;
}

.module-tile:hover{ box-shadow: var(--shadow-hover); transform: translateY(-3px); }
.module-tile.is-soon{ opacity: 0.6; cursor: default; }
.module-tile.is-soon:hover{ transform: none; }

.module-tile-icon{ font-size: 34px; margin-bottom: 10px; }
.module-tile-title{ font-weight: 800; font-size: 18px; margin-bottom: 6px; }
.module-tile-meta{ color: var(--text-muted); font-size: 13px; margin-bottom: 14px; }

.module-tile-soon-badge{
  display: inline-block;
  background: var(--border);
  color: var(--text-muted);
  font-size: 11px;
  font-weight: 700;
  padding: 3px 10px;
  border-radius: 20px;
}

.module-progress-bar-bg{
  background: var(--border);
  border-radius: 20px;
  height: 8px;
  overflow: hidden;
}

.module-progress-bar-fill{
  height: 100%;
  border-radius: 20px;
  background: var(--tile-color, var(--primary));
}

.module-progress-label{
  margin-top: 6px;
  font-size: 12px;
  color: var(--text-muted);
}

/* ===== BREADCRUMB ===== */
.breadcrumb{
  font-size: 13px;
  color: var(--text-muted);
  margin-bottom: 14px;
}

.breadcrumb a{ color: var(--primary-dark); text-decoration: none; font-weight: 600; }
.breadcrumb a:hover{ text-decoration: underline; }
.breadcrumb-sep{ margin: 0 6px; }
.breadcrumb-current{ color: var(--text-main); font-weight: 600; }

/* ===== PAGE TITLE ===== */
.page-title{ font-size: 24px; font-weight: 900; margin: 0 0 4px; }
.page-subtitle{ color: var(--text-muted); margin: 0 0 20px; font-size: 14px; }
.empty-text{ color: var(--text-muted); text-align: center; padding: 30px 0; }

/* ===== STATE BOX (loading / error) ===== */
.state-box{
  text-align: center;
  padding: 50px 20px;
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  margin-top: 10px;
}

.spinner{
  width: 40px;
  height: 40px;
  margin: 0 auto 16px;
  border: 4px solid var(--border);
  border-top-color: var(--primary);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
}

@keyframes spin{ to{ transform: rotate(360deg); } }

.error-box{ border: 1px solid #f5c2c7; }
.error-title{ font-weight: 700; color: var(--danger); font-size: 18px; margin-bottom: 8px; }

/* ===== BUTTONS ===== */
.btn{
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-family: inherit;
  font-size: 15px;
  font-weight: 600;
  border: none;
  border-radius: var(--radius-sm);
  padding: 12px 20px;
  cursor: pointer;
  text-decoration: none;
  transition: transform 0.12s ease, box-shadow 0.12s ease, background 0.12s ease;
  -webkit-tap-highlight-color: transparent;
}

.btn:active{ transform: scale(0.97); }

.btn-primary{
  background: linear-gradient(135deg, var(--primary), var(--primary-dark));
  color: #fff;
  box-shadow: 0 4px 12px rgba(230, 90, 40, 0.3);
}

.btn-primary:hover{ box-shadow: 0 6px 18px rgba(230, 90, 40, 0.4); }

.btn-secondary{
  background: var(--card-bg);
  color: var(--primary-dark);
  border: 2px solid var(--primary);
}

.btn-secondary:hover{ background: var(--tint-bg); }

.btn-ghost{
  background: transparent;
  color: var(--primary-dark);
  border: 1px dashed var(--primary);
  padding: 8px 16px;
  font-size: 14px;
}

.btn-large{ padding: 16px 24px; font-size: 16px; }
.btn-small{ padding: 8px 16px; font-size: 13px; }
.btn:disabled{ opacity: 0.5; cursor: not-allowed; }

/* ===== ẢNH TÀI LIỆU (module Đọc tài liệu) ===== */
.practice-image-card{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 16px;
  margin-bottom: 18px;
  text-align: center;
}

.material-type-badge{
  display: inline-block;
  background: var(--mod-material);
  color: #fff;
  font-size: 12px;
  font-weight: 700;
  padding: 4px 12px;
  border-radius: 20px;
  margin-bottom: 12px;
}

.practice-image{
  max-width: 100%;
  height: auto;
  border-radius: var(--radius-md);
  display: block;
  margin: 0 auto;
}

/* ===== CARD GRID: Book / Lesson ===== */
.card-grid{
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 14px;
}

.nav-card{
  display: block;
  background: var(--card-bg);
  border-radius: var(--radius-md);
  padding: 20px 16px;
  box-shadow: var(--shadow);
  border: 1px solid var(--border);
  text-decoration: none;
  color: var(--text-main);
  transition: box-shadow 0.15s ease, transform 0.15s ease;
}

.nav-card:hover{ box-shadow: var(--shadow-hover); transform: translateY(-2px); }

.nav-card-icon{ font-size: 28px; margin-bottom: 8px; }
.nav-card-title{ font-weight: 800; font-size: 17px; line-height: 1.5; margin-bottom: 6px; }
.nav-card-title .zh{ font-size: 19px; }
.nav-card-meta{ color: var(--text-muted); font-size: 13px; }

/* ===== AUDIO CARD ===== */
.audio-card{
  background: var(--card-bg);
  border-radius: var(--radius-md);
  padding: 18px 16px;
  box-shadow: var(--shadow);
  border: 1px solid var(--border);
  text-align: center;
}

.audio-card-icon{ font-size: 30px; margin-bottom: 6px; }
.audio-card-title{ font-weight: 800; font-size: 16px; margin-bottom: 4px; }
.audio-card-meta{ color: var(--text-muted); font-size: 12px; margin-bottom: 14px; }

/* ===== QUIZ ===== */
.quiz-audio-card{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 18px 16px;
  margin-bottom: 18px;
}

/* ===== CUSTOM AUDIO PLAYER (nút phát to) ===== */
.audio-player-custom{
  display: flex;
  align-items: center;
  gap: 16px;
  margin-bottom: 12px;
}

.audio-play-btn{
  flex-shrink: 0;
  width: 68px;
  height: 68px;
  border-radius: 50%;
  border: none;
  background: linear-gradient(135deg, var(--primary), var(--primary-dark));
  color: #fff;
  font-size: 26px;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  box-shadow: 0 4px 14px rgba(230, 90, 40, 0.35);
  transition: transform 0.12s ease, box-shadow 0.12s ease;
}

.audio-play-btn:hover{ box-shadow: 0 6px 18px rgba(230, 90, 40, 0.45); }
.audio-play-btn:active{ transform: scale(0.94); }

/* Icon play lệch tâm 1 chút để trông cân đối hơn khi dùng ký tự ▶ */
.audio-play-btn .icon-play{ margin-left: 4px; }

.audio-progress-wrap{ flex: 1; min-width: 0; }

.audio-seek{
  width: 100%;
  accent-color: var(--primary);
  height: 6px;
  cursor: pointer;
}

.audio-time{
  margin-top: 6px;
  font-size: 13px;
  color: var(--text-muted);
  font-variant-numeric: tabular-nums;
}

.dialogue-box{
  background: var(--tint-bg);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  padding: 16px;
  margin-top: 12px;
}

.dialogue-box p{
  margin: 0;
  white-space: pre-line;
  font-size: 22px;
  line-height: 2;
}

.question-block{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 18px 16px;
  margin-bottom: 16px;
  position: relative;
}

.question-number{
  display: inline-block;
  background: var(--primary);
  color: #fff;
  font-size: 12px;
  font-weight: 700;
  padding: 4px 12px;
  border-radius: 20px;
  margin-bottom: 10px;
}

.question-text{
  font-size: 24px;
  font-weight: 700;
  line-height: 1.6;
  margin: 0 0 16px;
}

.options-list{
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.option-btn{
  display: flex;
  align-items: center;
  gap: 14px;
  width: 100%;
  text-align: left;
  background: var(--card-bg);
  border: 2px solid var(--border);
  border-radius: var(--radius-md);
  padding: 16px 18px;
  font-family: 'DFKai-SB', 'BiauKai', 'KaiTi', 'cwTeX Q Kai', 'Noto Serif TC', serif;
  font-size: 20px;
  line-height: 1.5;
  cursor: pointer;
  transition: border-color 0.15s ease, background 0.15s ease;
}

.option-btn:hover{ border-color: var(--secondary); }

.option-btn.selected{ border-color: var(--primary); background: var(--tint-bg); }

.option-btn .option-letter{
  flex-shrink: 0;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  background: var(--border);
  color: var(--text-main);
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-family: 'Be Vietnam Pro', sans-serif;
}

.option-btn.selected .option-letter{ background: var(--primary); color: #fff; }

.option-btn.correct{ border-color: var(--success); background: var(--success-bg); }
.option-btn.correct .option-letter{ background: var(--success); color: #fff; }

.option-btn.wrong{ border-color: var(--danger); background: var(--danger-bg); }
.option-btn.wrong .option-letter{ background: var(--danger); color: #fff; }

.option-btn:disabled{ cursor: default; }

.question-result-tag{
  margin-top: 12px;
  font-weight: 700;
  font-size: 14px;
  padding: 8px 12px;
  border-radius: var(--radius-sm);
}

.question-result-tag.is-correct{ background: var(--success-bg); color: var(--success); }
.question-result-tag.is-wrong{ background: var(--danger-bg); color: var(--danger); }

/* ===== GIẢI THÍCH ĐÁP ÁN (ẩn/hiện) ===== */
.explanation-toggle-btn{
  margin-top: 10px;
}

.explanation-box{
  margin-top: 10px;
  background: var(--tint-bg);
  border-radius: var(--radius-sm);
  padding: 14px 16px;
  font-size: 15px;
  line-height: 1.7;
}

/* ===== ⭐ ĐÁNH DẤU CÂU KHÓ ===== */
.question-header-row{
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 10px;
}

.bookmark-btn{
  flex-shrink: 0;
  background: none;
  border: none;
  font-size: 24px;
  cursor: pointer;
  line-height: 1;
  padding: 2px;
  color: var(--text-muted);
}

.bookmark-btn.is-bookmarked{ color: #f5b301; }

/* ===== RESULT SUMMARY ===== */
.result-summary-card{
  background: linear-gradient(135deg, var(--primary), var(--secondary));
  color: #fff;
  border-radius: var(--radius-lg);
  padding: 24px 16px;
  text-align: center;
  margin-bottom: 18px;
  box-shadow: var(--shadow-hover);
}

.result-summary-score{ font-size: 36px; font-weight: 900; }
.result-summary-percent{ font-size: 18px; font-weight: 700; opacity: 0.95; }
.result-summary-label{ font-size: 13px; opacity: 0.9; margin-top: 4px; }

/* ===== SUBMIT / ACTION BAR ===== */
.quiz-submit-bar{
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin-top: 10px;
}

.quiz-submit-bar .btn{ width: 100%; }

.save-warning{
  background: var(--danger-bg);
  color: var(--danger);
  border-radius: var(--radius-sm);
  padding: 10px 14px;
  font-size: 13px;
  margin-top: 10px;
  text-align: center;
}

/* ===== BẢNG TRẮNG DẠY HỌC (whiteboard) ===== */
.whiteboard-panel{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 14px 16px 16px;
  margin-bottom: 18px;
}

.whiteboard-toolbar{
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
  margin-bottom: 10px;
}

.whiteboard-tool-btn{
  background: var(--card-bg);
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 6px 14px;
  font-size: 13px;
  font-weight: 600;
  font-family: inherit;
  color: var(--text-main);
  cursor: pointer;
}

.whiteboard-tool-btn.active{
  background: var(--primary);
  border-color: var(--primary);
  color: #fff;
}

.whiteboard-toolbar-sep{
  width: 1px;
  align-self: stretch;
  background: var(--border);
  margin: 0 2px;
}

.whiteboard-color{
  width: 28px;
  height: 28px;
  border-radius: 50%;
  border: 2px solid var(--border);
  cursor: pointer;
  padding: 0;
}

.whiteboard-color.active{ border-color: var(--text-main); box-shadow: 0 0 0 2px #fff inset; }

.whiteboard-spacer{ flex: 1; }

.whiteboard-hint{
  font-size: 12px;
  color: var(--text-muted);
  margin: 0 0 8px;
}

.whiteboard-canvas-wrap{
  position: relative;
  border: 2px dashed var(--border);
  border-radius: var(--radius-md);
  overflow: hidden;
  background: var(--card-bg);
  touch-action: none;
}

.whiteboard-canvas{
  display: block;
  width: 100%;
  height: 320px;
  cursor: crosshair;
}

.whiteboard-text-input{
  position: absolute;
  z-index: 5;
  min-width: 140px;
  font-size: 20px;
  font-family: 'cwTeX Q Kai', 'Noto Sans TC', sans-serif;
  border: 1px dashed var(--primary);
  background: rgba(255, 255, 255, 0.92);
  padding: 2px 4px;
  outline: none;
}

/* Lớp chứa các chữ đã viết — mặc định "trong suốt" với thao tác chuột để
   không cản việc vẽ; chỉ bật click khi đang ở chế độ Gõ chữ / Đánh dấu. */
.whiteboard-text-layer{
  position: absolute;
  inset: 0;
  pointer-events: none;
}

.whiteboard-text-obj{
  position: absolute;
  pointer-events: none;
  font-size: 20px;
  line-height: 1.3;
  padding: 1px 3px;
  border-radius: 3px;
}

.wb-mode-text .whiteboard-text-obj,
.wb-mode-highlight .whiteboard-text-obj{
  pointer-events: auto;
  cursor: pointer;
}

.whiteboard-text-obj:hover{ outline: 1px dashed var(--primary); }

.whiteboard-text-obj.highlighted .whiteboard-text-content{
  background: rgba(255, 224, 71, 0.75);
  box-decoration-break: clone;
  -webkit-box-decoration-break: clone;
}

/* ===== NAME MODAL ===== */
.modal-overlay{
  position: fixed;
  inset: 0;
  background: rgba(43, 35, 32, 0.55);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;
  z-index: 100;
}

.modal-box{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  padding: 24px 20px;
  max-width: 380px;
  width: 100%;
  box-shadow: 0 12px 32px rgba(0,0,0,0.25);
}

.modal-title{ margin: 0 0 8px; font-size: 19px; }
.modal-desc{ margin: 0 0 16px; font-size: 13px; color: var(--text-muted); }

.modal-input{
  width: 100%;
  padding: 12px 14px;
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: 15px;
  font-family: inherit;
  margin-bottom: 16px;
}

.modal-input:focus{ outline: none; border-color: var(--primary); }

.modal-actions{ display: flex; gap: 10px; }
.modal-actions .btn{ flex: 1; }

/* ===== LOGIN (đăng nhập MSSV) ===== */
.login-card{
  max-width: 380px;
  margin: 40px auto 0;
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 28px 24px;
  text-align: center;
}

.login-card .page-title{ margin-bottom: 6px; }
.login-card .page-subtitle{ margin-bottom: 18px; }
.login-card .modal-input{ text-align: center; font-size: 17px; }
.login-card .btn-large{ width: 100%; }

.login-error{
  background: var(--danger-bg);
  color: var(--danger);
  border-radius: var(--radius-sm);
  padding: 10px 14px;
  font-size: 13px;
  margin-bottom: 14px;
}

/* ===== BÁO CÁO GIÁO VIÊN (#/admin) ===== */
.admin-table-wrap{
  background: var(--card-bg);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow);
  padding: 8px;
  overflow-x: auto;
}

.admin-table{
  width: 100%;
  border-collapse: collapse;
  font-size: 14px;
  white-space: nowrap;
}

.admin-table th,
.admin-table td{
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid var(--border);
}

.admin-table th{
  color: var(--text-muted);
  font-weight: 700;
  font-size: 12px;
  text-transform: uppercase;
}

.admin-table tbody tr:hover{ background: var(--bg); }
.admin-table tbody tr:last-child td{ border-bottom: none; }

/* ===== RESPONSIVE ===== */
@media (min-width: 640px){
  .site-title{ font-size: 30px; }
  .quiz-submit-bar{ flex-direction: row; }
  .quiz-submit-bar .btn{ flex: 1; }
}

@media (max-width: 480px){
  .header-inner{ flex-direction: column; align-items: flex-start; }
}

/* ===== MOBILE (< 600px) — tối ưu riêng cho điện thoại ===== */
@media (max-width: 600px){
  /* 1. Header thấp hơn, gọn hơn */
  .site-header{ padding: 12px 0; }
  .header-inner{ flex-direction: row; flex-wrap: wrap; align-items: center; }
  .site-title{ font-size: 30px; }
  .site-subtitle{ font-size: 15px; margin-top: 2px; }
  .header-actions{ gap: 8px; }
  .theme-toggle-btn{ width: 34px; height: 34px; font-size: 16px; }
  .student-badge{ font-size: 12px; padding: 6px 10px; }

  /* 2. Breadcrumb — cuộn ngang gọn, không xuống nhiều dòng */
  .breadcrumb{
    font-size: 15px;
    white-space: nowrap;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
    display: block;
    padding-bottom: 4px;
    margin-bottom: 10px;
  }
  .breadcrumb::-webkit-scrollbar{ height: 3px; }

  /* 3. Question card — padding giảm, bo góc vừa phải, chữ to dễ đọc */
  .question-block{
    padding: 14px 12px;
    border-radius: var(--radius-md);
  }
  .question-text{
    font-size: 32px;
    line-height: 1.55;
    margin: 0 0 14px;
  }

  /* 4. Đáp án — chữ to, nút vừa tay, khoảng cách gọn hơn */
  .options-list{ gap: 8px; }
  .option-btn{
    padding: 17px 16px;
    font-size: 30px;
    gap: 12px;
  }
  .option-btn .option-letter{
    width: 44px;
    height: 44px;
    font-size: 18px;
  }

  /* 5. Nút Nộp đáp án — to vừa, dính đáy màn hình nhưng không che nội dung
     (position: sticky, không phải fixed — luôn nằm đúng vị trí trong luồng
     trang, chỉ "dính" khi cuộn tới, không đè lên nội dung phía trên nó). */
  .quiz-submit-bar{
    margin-top: 14px;
    position: sticky;
    bottom: 0;
    background: var(--bg);
    padding: 10px 0 14px;
    z-index: 5;
  }
  .quiz-submit-bar .btn-large{
    padding: 18px;
    font-size: 25px;
  }
}