:root {
    --color-primary: #667eea;
    --color-secondary: #764ba2;
    --text-dark: #2d3748;
    --bg-light: rgba(255, 255, 255, 0.95);
    --shadow-base: 0 10px 30px rgba(0, 0, 0, 0.1);
    --shadow-hover: 0 20px 40px rgba(0, 0, 0, 0.15);
    
    /* 各年级色彩基调 */
    --grade1-hue: 330; 
    --grade2-hue: 210; 
    --grade3-hue: 39;  
    --grade4-hue: 125; 
    --grade5-hue: 279; 
    --grade6-hue: 190; 
}

* { margin: 0; padding: 0; box-sizing: border-box; }

html, body {
    height: 100%;
    scroll-behavior: smooth;
    overflow-x: hidden;
    /* 防止移动端下拉刷新和手势冲突 */
    overscroll-behavior-y: none;
}

body {
    font-family: 'Noto Sans SC', -apple-system, BlinkMacSystemFont, sans-serif;
    background: linear-gradient(135deg, 
        hsla(var(--grade1-hue), 80%, 90%, 0.2),
        hsla(var(--grade2-hue), 80%, 90%, 0.2) 30%,
        hsla(var(--grade4-hue), 80%, 90%, 0.2) 70%,
        hsla(var(--grade6-hue), 80%, 90%, 0.2) 100%
    );
    background-attachment: fixed;
    color: var(--text-dark);
    line-height: 1.6;
}

.container {
    width: 100%;
    max-width: 1400px;
    margin: 0 auto;
    padding: clamp(1rem, 3vw, 2rem);
}

/* --- Header 头 --- */
.app-header {
    text-align: center;
    margin-bottom: clamp(1.5rem, 4vw, 2.5rem);
    padding: clamp(1.5rem, 4vw, 2.5rem);
    background: var(--bg-light);
    border-radius: clamp(1rem, 3vw, 1.5rem);
    box-shadow: var(--shadow-base);
    position: relative;
    backdrop-filter: blur(20px);
}

.app-header h1 {
    font-size: clamp(1.6rem, 4vw, 2.8rem);
    margin-bottom: 0.5rem;
    font-weight: 700;
    color: var(--text-dark);
}

.app-header p { font-size: clamp(0.9rem, 2vw, 1.2rem); opacity: 0.8; }

.back-btn {
    position: absolute;
    left: clamp(1rem, 3vw, 2rem); top: 50%; transform: translateY(-50%);
    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
    color: white; border: none; padding: 0.6rem 1.2rem; border-radius: 50px;
    cursor: pointer; font-size: clamp(0.9rem, 2vw, 1rem); transition: all 0.3s;
    display: flex; align-items: center; gap: 0.5rem; opacity: 0; pointer-events: none;
}
.back-btn.active { opacity: 1; pointer-events: auto; }
.back-btn svg { width: 1.2rem; height: 1.2rem; }
.back-btn:hover { transform: translateY(-50%) scale(1.05); }

/* 年级主题按钮颜色 */
.back-btn.grade-1 { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 65%), hsl(var(--grade1-hue), 80%, 50%)); }
.back-btn.grade-2 { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 65%), hsl(var(--grade2-hue), 80%, 50%)); }
.back-btn.grade-3 { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 65%), hsl(var(--grade3-hue), 80%, 50%)); }
.back-btn.grade-4 { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 65%), hsl(var(--grade4-hue), 80%, 50%)); }
.back-btn.grade-5 { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 65%), hsl(var(--grade5-hue), 80%, 50%)); }
.back-btn.grade-6 { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 65%), hsl(var(--grade6-hue), 80%, 50%)); }

.view-section { display: none; animation: fadeIn 0.4s ease-out forwards; }
.view-section.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(10px); } to { opacity: 1; transform: translateY(0); } }

/* --- 切换器 --- */
.volume-toggle-container { display: flex; justify-content: center; margin-bottom: 2.5rem; }
.volume-toggle { position: relative; display: flex; background: #e2e8f0; border-radius: 50px; padding: 5px; box-shadow: inset 0 2px 5px rgba(0,0,0,0.1); }
.volume-toggle input[type="radio"] { display: none; }
.toggle-btn { position: relative; z-index: 2; padding: 0.8rem 2.5rem; font-size: 1.1rem; font-weight: 600; color: #64748b; cursor: pointer; transition: color 0.3s ease; text-align: center; }
.toggle-slider { position: absolute; top: 5px; left: 5px; width: calc(50% - 5px); height: calc(100% - 10px); background: linear-gradient(135deg, var(--color-primary), var(--color-secondary)); border-radius: 50px; transition: transform 0.3s cubic-bezier(0.4, 0.0, 0.2, 1); box-shadow: 0 4px 10px rgba(102, 126, 234, 0.4); }
#vol-s:checked ~ .toggle-btn:nth-of-type(1) { color: white; }
#vol-x:checked ~ .toggle-btn:nth-of-type(2) { color: white; }
#vol-x:checked ~ .toggle-slider { transform: translateX(100%); }

/* --- 首页网格 --- */
.homepage-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: clamp(1rem, 3vw, 2.5rem); }
.grade-card { position: relative; border-radius: 1.5rem; padding: clamp(2rem, 5vw, 3rem) 1.5rem; text-align: center; cursor: pointer; transition: all 0.3s; color: white; box-shadow: var(--shadow-base); overflow: hidden; }
.grade-card:hover { transform: translateY(-8px); box-shadow: var(--shadow-hover); }
.grade-card:nth-child(1) { background: linear-gradient(135deg, hsl(var(--grade1-hue), 80%, 70%), hsl(var(--grade1-hue), 80%, 55%)); }
.grade-card:nth-child(2) { background: linear-gradient(135deg, hsl(var(--grade2-hue), 80%, 70%), hsl(var(--grade2-hue), 80%, 55%)); }
.grade-card:nth-child(3) { background: linear-gradient(135deg, hsl(var(--grade3-hue), 80%, 70%), hsl(var(--grade3-hue), 80%, 55%)); }
.grade-card:nth-child(4) { background: linear-gradient(135deg, hsl(var(--grade4-hue), 80%, 70%), hsl(var(--grade4-hue), 80%, 55%)); }
.grade-card:nth-child(5) { background: linear-gradient(135deg, hsl(var(--grade5-hue), 80%, 70%), hsl(var(--grade5-hue), 80%, 55%)); }
.grade-card:nth-child(6) { background: linear-gradient(135deg, hsl(var(--grade6-hue), 80%, 70%), hsl(var(--grade6-hue), 80%, 55%)); }
.grade-number { font-size: clamp(3rem, 8vw, 5rem); font-weight: 800; line-height: 1; margin-bottom: 1rem; }
.grade-title { font-size: clamp(1.4rem, 3vw, 2rem); margin-bottom: 0.5rem; }
.grade-desc { font-size: 1rem; opacity: 0.9; background: rgba(0,0,0,0.1); display: inline-block; padding: 4px 12px; border-radius: 20px;}

/* --- 课程内容页 --- */
.grade-content { background: var(--bg-light); border-radius: 1.5rem; padding: clamp(1.5rem, 4vw, 2.5rem); box-shadow: var(--shadow-base); }
.unit-title { font-size: clamp(1.1rem, 2vw, 1.25rem); margin: 1rem 0; padding: 1rem 1.5rem; border-radius: 1rem; cursor: pointer; background: rgba(102, 126, 234, 0.1); font-weight: 500; display: flex; align-items: center; transition: all 0.3s; position: relative; }
.unit-title::after { content: '▼'; position: absolute; right: 1.5rem; font-size: 0.8em; transition: transform 0.3s ease; opacity: 0.7; }
.unit-title.active { background: rgba(102, 126, 234, 0.2); font-weight: 700; }
.unit-title.active::after { transform: rotate(180deg); }
.unit-icon { width: 32px; height: 32px; background: var(--color-primary); color: white; border-radius: 50%; display: inline-flex; justify-content: center; align-items: center; margin-right: 1rem; flex-shrink: 0; font-size: 1rem; }

.video-grid { display: none; grid-template-columns: repeat(auto-fit, minmax(min(100%, 300px), 1fr)); gap: clamp(1rem, 3vw, 2rem); margin-bottom: 2rem; }
.video-grid.active { display: grid; }
.video-card { background: rgba(255, 255, 255, 0.9); border-radius: 1rem; padding: 1rem; box-shadow: 0 5px 15px rgba(0,0,0,0.05); border: 1px solid rgba(0,0,0,0.05); }
.video-title { font-weight: 600; margin-bottom: 1rem; text-align: center; font-size: 1.05rem;}

/* --- 视频播放器 (自适应模式) --- */
.video-wrapper { position: relative; width: 100%; border-radius: 0.8rem; overflow: hidden; background: #000; aspect-ratio: 16 / 9; user-select: none; }
.video-wrapper video { width: 100%; height: 100%; object-fit: contain; outline: none; display: block; }

/* PC 端：隐藏自定义控件层 */
.pc-player .video-placeholder, .pc-player .custom-controls, .pc-player .gesture-toast { display: none !important; }

/* 移动端：自定义控件层 */
.mobile-player .video-placeholder { position: absolute; top: 0; left: 0; width: 100%; height: 100%; background: rgba(0,0,0,0.2); display: flex; justify-content: center; align-items: center; cursor: pointer; transition: opacity 0.3s; z-index: 2; pointer-events: none; }
.mobile-player .video-placeholder.hidden { opacity: 0; }
.play-icon { width: 3rem; height: 3rem; background: rgba(255,255,255,0.9); border-radius: 50%; display: flex; justify-content: center; align-items: center; color: var(--color-primary); font-size: 1.2rem; padding-left: 3px; transition: transform 0.2s; pointer-events: auto; }

/* 移动端手势 Toast 提示层 */
.gesture-toast {
    position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%);
    background: rgba(0,0,0,0.7); color: white; padding: 12px 24px; border-radius: 50px;
    font-size: 1.1rem; font-weight: 600; pointer-events: none; opacity: 0; z-index: 20;
    transition: opacity 0.2s; text-align: center; letter-spacing: 1px; backdrop-filter: blur(5px);
}
.gesture-toast.active { opacity: 1; }

/* 控制栏 */
.custom-controls { position: absolute; bottom: 0; left: 0; right: 0; background: linear-gradient(to top, rgba(0,0,0,0.85) 0%, rgba(0,0,0,0) 100%); padding: 10px 15px; display: flex; flex-direction: column; gap: 8px; opacity: 0; transition: opacity 0.3s ease; z-index: 10; }
.mobile-player.show-controls .custom-controls, .mobile-player.paused .custom-controls { opacity: 1; }

.progress-container { width: 100%; height: 6px; background: rgba(255,255,255,0.3); cursor: pointer; border-radius: 3px; position: relative; }
.progress-filled { width: 0%; height: 100%; background: var(--color-primary); border-radius: 3px; pointer-events: none; transition: width 0.1s linear;}

.controls-row { display: flex; align-items: center; gap: 12px; }
.control-btn { background: none; border: none; color: white; font-size: 1.2rem; cursor: pointer; display: flex; align-items: center; justify-content: center; transition: color 0.2s; font-weight: bold;}
.control-btn:hover { color: #e2e8f0; }

.speed-btn { font-size: 0.95rem; font-family: sans-serif; min-width: 40px;}
.pip-btn { margin-left: auto; font-size: 1.1rem; }

.time-display { color: white; font-size: 0.85rem; font-family: monospace; flex-grow: 1; pointer-events: none; margin-left: 5px; }

.coming-soon { text-align: center; padding: 3rem 1rem; color: #666; font-size: 1.2rem; background: #f8f9fa; border-radius: 1rem; border: 2px dashed #ddd; }

/* 主题颜色覆写 */
.theme-grade-1 .unit-title.active { background: hsla(var(--grade1-hue), 80%, 90%, 0.8); }
.theme-grade-1 .unit-icon, .theme-grade-1 .play-icon { background: hsl(var(--grade1-hue), 80%, 60%); color:#fff; }
.theme-grade-1 .progress-filled { background: hsl(var(--grade1-hue), 80%, 60%); }

.theme-grade-2 .unit-title.active { background: hsla(var(--grade2-hue), 80%, 90%, 0.8); }
.theme-grade-2 .unit-icon, .theme-grade-2 .play-icon, .theme-grade-2 .progress-filled { background: hsl(var(--grade2-hue), 80%, 60%); color:#fff; }
.theme-grade-3 .unit-icon, .theme-grade-3 .play-icon, .theme-grade-3 .progress-filled { background: hsl(var(--grade3-hue), 80%, 60%); color:#fff; }
.theme-grade-4 .unit-icon, .theme-grade-4 .play-icon, .theme-grade-4 .progress-filled { background: hsl(var(--grade4-hue), 80%, 60%); color:#fff; }
.theme-grade-5 .unit-icon, .theme-grade-5 .play-icon, .theme-grade-5 .progress-filled { background: hsl(var(--grade5-hue), 80%, 60%); color:#fff; }
.theme-grade-6 .unit-icon, .theme-grade-6 .play-icon, .theme-grade-6 .progress-filled { background: hsl(var(--grade6-hue), 80%, 60%); color:#fff; }

/* 响应式 */
@media (max-width: 900px) { .homepage-grid { grid-template-columns: repeat(2, 1fr); } }
@media (max-width: 600px) {
    .app-header { display: flex; flex-direction: column; padding-top: 1rem; }
    .back-btn { position: relative; left: 0; top: 0; transform: none; margin-bottom: 1rem; width: max-content; margin-inline: auto; }
    .back-btn:hover { transform: scale(1.05); }
    .toggle-btn { padding: 0.6rem 1.5rem; font-size: 1rem; }
    .homepage-grid { grid-template-columns: 1fr; }
    .video-grid { grid-template-columns: 1fr; }
}