/* ========= CSS 变量定义 ========= */
:root {
    /* 主题色 */
    --color-primary: #1e3c72;
    --color-primary-hover: #143157;
    --color-primary-light: #60a5fa;
    
    /* 背景色 */
    --bg-body: #f9fafb;
    --bg-card: #ffffff;
    --bg-hero: #e8edf3;
    --bg-nav: #ffffff;
    --bg-code: #1e293b;
    --bg-code-inline: #f1f5f9;
    --bg-blockquote: #f8fafc;
    --bg-hover: #f0f7ff;
    --bg-tab-active: #eef2ff;
    --bg-badge: #eef2ff;
    --bg-other-hover: #fefce8;
    
    /* 文字色 */
    --text-primary: #1f2937;
    --text-secondary: #6b7280;
    --text-muted: #94a3b8;
    --text-heading: #1e293b;
    --text-heading-dark: #0f2b3d;
    --text-content: #2d3a4b;
    --text-link: #2d6a9f;
    --text-link-hover: #1e3c72;
    --text-code: #c2410c;
    --text-code-block: #e2e8f0;
    --text-footer: #6c86a3;
    --text-comment-author: #1e3c72;
    --text-comment-anonymous: #64748b;
    
    /* 边框色 */
    --border-light: #edf2f7;
    --border-medium: #e5e7eb;
    --border-accent: #bdd3ff;
    --border-blockquote: #bdd3ff;
    --border-other: #d9e6f5;
    --border-other-hover: #eab308;
    --border-nav: #e2e8f0;
    --border-footer: #e9eef3;
    
    /* 阴影 */
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.05);
    --shadow-card: 0 4px 14px rgba(0, 0, 0, 0.02);
    --shadow-hover: 0 12px 20px -12px rgba(0, 0, 0, 0.15);
    --shadow-btn: 0 4px 12px rgba(30, 60, 114, 0.3);
    --shadow-btn-hover: 0 6px 16px rgba(30, 60, 114, 0.4);
    
    /* Logo 渐变 */
    --logo-gradient-start: #1e3c72;
    --logo-gradient-end: #2b4c7c;
}

/* 深色模式 */
[data-theme="dark"] {
    /* 主题色 */
    --color-primary: #60a5fa;
    --color-primary-hover: #3b82f6;
    --color-primary-light: #93c5fd;
    
    /* 背景色 */
    --bg-body: #0f172a;
    --bg-card: #1e293b;
    --bg-hero: #1e293b;
    --bg-nav: #1e293b;
    --bg-code: #0f172a;
    --bg-code-inline: #334155;
    --bg-blockquote: #334155;
    --bg-hover: #334155;
    --bg-tab-active: #334155;
    --bg-badge: #334155;
    --bg-other-hover: #334155;
    
    /* 文字色 */
    --text-primary: #f1f5f9;
    --text-secondary: #94a3b8;
    --text-muted: #64748b;
    --text-heading: #f1f5f9;
    --text-heading-dark: #e2e8f0;
    --text-content: #cbd5e1;
    --text-link: #93c5fd;
    --text-link-hover: #60a5fa;
    --text-code: #fbbf24;
    --text-code-block: #e2e8f0;
    --text-footer: #94a3b8;
    --text-comment-author: #60a5fa;
    --text-comment-anonymous: #94a3b8;
    
    /* 边框色 */
    --border-light: #334155;
    --border-medium: #475569;
    --border-accent: #475569;
    --border-blockquote: #60a5fa;
    --border-other: #475569;
    --border-other-hover: #fbbf24;
    --border-nav: #475569;
    --border-footer: #334155;
    
    /* 阴影 */
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.3);
    --shadow-card: 0 4px 14px rgba(0, 0, 0, 0.3);
    --shadow-hover: 0 12px 20px -12px rgba(0, 0, 0, 0.5);
    --shadow-btn: 0 4px 12px rgba(96, 165, 250, 0.3);
    --shadow-btn-hover: 0 6px 16px rgba(96, 165, 250, 0.4);
    
    /* Logo 渐变 */
    --logo-gradient-start: #60a5fa;
    --logo-gradient-end: #93c5fd;
}

/* 平滑过渡 */
body,
.hero,
.content-card,
.lesson-item,
.concept-card,
.other-item,
.nav-btn,
.back-to-top,
footer {
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

/* ========= 公共基础 ========= */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', sans-serif;
    background-color: var(--bg-body);
    color: var(--text-primary);
    line-height: 1.6;
}

.container {
    max-width: 1280px;
    margin: 0 auto;
    padding: 0 24px;
}

/* ========= 导航栏 ========= */
.navbar {
    background-color: var(--bg-nav);
    box-shadow: var(--shadow-sm);
    position: sticky;
    top: 0;
    z-index: 50;
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease, transform 0.3s ease;
}

.navbar--hidden {
    transform: translateY(-100%);
}

.nav-inner {
    display: flex;
    align-items: center;
    justify-content: space-between;
    flex-wrap: wrap;
    padding: 16px 24px;
    max-width: 1280px;
    margin: 0 auto;
}

.logo {
    font-size: 1.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--logo-gradient-start), var(--logo-gradient-end));
    -webkit-background-clip: text;
    background-clip: text;
    color: transparent;
    letter-spacing: -0.3px;
}

.nav-links {
    display: flex;
    gap: 28px;
    list-style: none;
    font-weight: 500;
    align-items: center;
}

.nav-links a {
    text-decoration: none;
    color: var(--text-secondary);
    transition: color 0.2s;
}

.nav-links a:hover {
    color: var(--color-primary);
}

/* 二级导航下拉菜单 */
.nav-dropdown {
    position: relative;
}

.nav-dropdown-toggle {
    display: flex;
    align-items: center;
    gap: 2px;
}

.dropdown-arrow {
    transition: transform 0.2s;
}

.nav-dropdown:hover .dropdown-arrow {
    transform: rotate(180deg);
}

.nav-dropdown-menu {
    position: absolute;
    top: 100%;
    left: 50%;
    transform: translateX(-50%);
    background: var(--bg-card);
    border: 1px solid var(--border-nav);
    border-radius: 12px;
    box-shadow: var(--shadow-hover);
    min-width: 160px;
    padding: 8px;
    list-style: none;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.2s, visibility 0.2s, transform 0.2s;
    margin-top: 8px;
}

.nav-dropdown-menu::before {
    content: '';
    position: absolute;
    top: -6px;
    left: 50%;
    transform: translateX(-50%) rotate(45deg);
    width: 10px;
    height: 10px;
    background: var(--bg-card);
    border-left: 1px solid var(--border-nav);
    border-top: 1px solid var(--border-nav);
}

.nav-dropdown:hover .nav-dropdown-menu {
    opacity: 1;
    visibility: visible;
}

.nav-dropdown-menu li {
    margin: 0;
    padding: 0;
}

.nav-dropdown-menu a {
    display: block;
    padding: 8px 16px;
    border-radius: 8px;
    color: var(--text-secondary);
    text-decoration: none;
    font-weight: 500;
    font-size: 0.9rem;
    transition: background-color 0.2s, color 0.2s;
    white-space: nowrap;
}

.nav-dropdown-menu a:hover {
    background: var(--bg-hover);
    color: var(--color-primary);
}

/* 主题切换按钮 */
.theme-toggle {
    background: none;
    border: none;
    padding: 8px;
    cursor: pointer;
    color: var(--text-secondary);
    font-size: 1.2rem;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 8px;
    transition: background-color 0.2s, color 0.2s;
}

.theme-toggle:hover {
    background-color: var(--bg-hover);
    color: var(--color-primary);
}

.theme-toggle svg {
    width: 20px;
    height: 20px;
}

/* ========= Hero 横幅 ========= */
.hero {
    background: var(--bg-hero);
    border-radius: 1.25rem;
    margin: 32px auto 40px;
    padding: 40px 32px;
    text-align: center;
    box-shadow: var(--shadow-card);
    border: 1px solid var(--border-light);
}

.hero h1 {
    font-size: 2rem;
    font-weight: 700;
    color: var(--text-heading);
    margin-bottom: 12px;
}

.hero-meta {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 12px;
    font-size: 0.9rem;
    color: var(--text-secondary);
    margin-bottom: 12px;
}

.hero-category {
    color: var(--color-primary);
    font-weight: 500;
}

.hero-badge {
    display: inline-block;
    background: var(--bg-badge);
    color: var(--color-primary);
    font-size: 0.8rem;
    padding: 4px 12px;
    border-radius: 40px;
    margin-bottom: 16px;
}

.hero-description {
    font-size: 1.1rem;
    color: var(--text-secondary);
    margin: 0 auto 12px;
}

.hero-source {
    margin-top: 16px;
}

.hero-source a {
    color: var(--text-link);
    text-decoration: none;
    font-size: 0.9rem;
    font-weight: 500;
    transition: color 0.2s;
}

.hero-source a:hover {
    color: var(--text-link-hover);
}

/* ========= Tab 样式 ========= */
.tab-bar {
    display: flex;
    gap: 8px;
    border-bottom: 2px solid var(--border-medium);
    margin-bottom: 32px;
    overflow-x: auto;
    overflow-y: hidden;
    scrollbar-width: none;
    -webkit-overflow-scrolling: touch;
}

.tab-bar::-webkit-scrollbar {
    display: none;
}

.tab-btn {
    background: none;
    border: none;
    padding: 10px 20px;
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-secondary);
    cursor: pointer;
    transition: all 0.2s;
    border-radius: 8px;
    white-space: nowrap;
    flex-shrink: 0;
}

.tab-btn:hover {
    background-color: var(--bg-hover);
    color: var(--text-primary);
}

.tab-btn.active {
    color: var(--color-primary);
    background-color: var(--bg-tab-active);
    box-shadow: inset 0 -2px 0 var(--color-primary);
}

.tab-pane {
    display: none;
    animation: fadeIn 0.2s ease;
}

.tab-pane.active-pane {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(6px); }
    to { opacity: 1; transform: translateY(0); }
}

/* ========= 列表卡片 — 课程列表（首页） ========= */
.lessons-list {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.lesson-item {
    background: var(--bg-card);
    border-radius: 12px;
    padding: 18px 24px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    transition: all 0.25s ease;
    border: 1px solid var(--border-light);
    box-shadow: var(--shadow-sm);
    cursor: pointer;
}

.lesson-item:hover {
    background-color: var(--bg-hover);
    border-color: var(--border-accent);
    transform: translateY(-2px);
    box-shadow: var(--shadow-hover);
}

.lesson-title {
    font-weight: 600;
    font-size: 1.05rem;
    color: var(--text-heading);
    display: flex;
    align-items: center;
    gap: 12px;
    flex-wrap: wrap;
}



.lesson-badge {
    background: var(--bg-badge);
    color: var(--color-primary);
    font-size: 0.75rem;
    padding: 4px 10px;
    border-radius: 40px;
    font-weight: 500;
}

.has-images-icon {
    margin-left: 8px;
    font-size: 1rem;
    opacity: 0.6;
}

/* 移动端：图片图标显示在标题后 */
.has-images-icon-mobile {
    display: none;
}

/* PC 端：图片图标显示在 meta 区域 */
.has-images-icon-desktop {
    display: inline;
}

@media (max-width: 640px) {
    .has-images-icon-mobile {
        display: inline;
    }
    .has-images-icon-desktop {
        display: none;
    }
}

.lesson-meta {
    color: var(--text-muted);
    font-size: 0.85rem;
    display: flex;
    gap: 16px;
    align-items: center;
}

/* ========= 列表卡片 — 核心概念网格（首页） ========= */
.concepts-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 24px;
}

.concept-card {
    background: var(--bg-card);
    border-radius: 16px;
    padding: 20px;
    border: 1px solid var(--border-light);
    transition: all 0.25s;
    cursor: pointer;
}

.concept-card:hover {
    background-color: var(--bg-hover);
    border-color: var(--border-accent);
    transform: translateY(-4px);
    box-shadow: var(--shadow-hover);
}

.concept-card h4 {
    font-size: 1.25rem;
    font-weight: 700;
    margin-bottom: 12px;
    color: var(--text-heading-dark);
}

.concept-card p {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

.concept-link {
    margin-top: 16px;
}

.concept-link a {
    color: var(--text-link);
    text-decoration: none;
    font-weight: 500;
}

.concept-link a:hover {
    color: var(--text-link-hover);
}

/* ========= 列表卡片 — 其他内容列表（首页） ========= */
.other-item {
    background: var(--bg-card);
    border-radius: 12px;
    padding: 16px 24px;
    margin-bottom: 12px;
    transition: all 0.2s;
    border-left: 4px solid var(--border-other);
    cursor: pointer;
}

.other-item:hover {
    background-color: var(--bg-other-hover);
    border-left-color: var(--border-other-hover);
    transform: translateX(6px);
}

.other-item-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.other-item-title {
    font-size: 1.05rem;
}

.other-item-desc {
    color: var(--text-secondary);
    margin-top: 6px;
    font-size: 0.9rem;
}

.other-item-date {
    color: var(--text-muted);
    font-size: 0.8rem;
    flex-shrink: 0;
    margin-left: 16px;
    white-space: nowrap;
}

/* 移动端：时间独占一行，在标题下面、描述上面 */
@media (max-width: 640px) {
    .other-item {
        display: flex;
        flex-direction: column;
    }
    .other-item-header {
        flex-direction: column;
        align-items: flex-start;
        gap: 4px;
    }
    .other-item-date {
        margin-left: 0;
        white-space: normal;
    }
}

/* ========= 文章详情 ========= */
.content-card {
    background: var(--bg-card);
    border-radius: 16px;
    padding: 40px 0;
    box-shadow: var(--shadow-card);
    border: 1px solid var(--border-light);
    margin-bottom: 32px;
}

.content-card-inner {
    max-width: 100%;
    padding: 0 40px;
    font-size: 1.1rem;
}

.content-card-inner p {
    margin-bottom: 1.2em;
    color: var(--text-content);
}

.content-card-inner h1 {
    font-size: 1.8rem;
    margin: 1.5em 0 0.8em;
    font-weight: 700;
    color: var(--text-heading-dark);
}

.content-card-inner h2 {
    font-size: 1.6rem;
    margin: 1.5em 0 0.8em;
    font-weight: 600;
    color: var(--text-heading-dark);
}

.content-card-inner h3 {
    font-size: 1.3rem;
    margin: 1.2em 0 0.6em;
    font-weight: 600;
    color: var(--text-heading);
}

.content-card-inner h4 {
    font-size: 1.1rem;
    margin: 1em 0 0.5em;
    font-weight: 600;
    color: var(--text-heading);
}

.content-card-inner blockquote {
    border-left: 4px solid var(--border-blockquote);
    background: var(--bg-blockquote);
    padding: 16px 24px;
    margin: 24px 0;
    border-radius: 12px;
    font-style: italic;
    color: var(--text-content);
}

.content-card-inner blockquote p {
    margin-bottom: 0.5em;
}

.content-card-inner blockquote p:last-child {
    margin-bottom: 0;
}

.content-card-inner img {
    max-width: 100%;
    height: auto;
    border-radius: 8px;
    margin: 16px 0;
}

.content-card-inner a {
    color: var(--text-link);
    text-decoration: none;
}

.content-card-inner a:hover {
    text-decoration: underline;
    color: var(--text-link-hover);
}

.content-card-inner ul,
.content-card-inner ol {
    margin: 1em 0;
    padding-left: 2em;
}

.content-card-inner li {
    margin-bottom: 0.5em;
    color: var(--text-content);
}

.content-card-inner table {
    width: 100%;
    border-collapse: collapse;
    margin: 1.5em 0;
}

.content-card-inner th,
.content-card-inner td {
    border: 1px solid var(--border-medium);
    padding: 10px 14px;
    text-align: left;
}

.content-card-inner th {
    background: var(--bg-blockquote);
    font-weight: 600;
}

.content-card-inner pre {
    background: var(--bg-code);
    color: var(--text-code-block);
    padding: 20px 24px;
    border-radius: 12px;
    overflow-x: auto;
    margin: 1.5em 0;
    font-size: 0.9rem;
    line-height: 1.6;
}

.content-card-inner code {
    background: var(--bg-code-inline);
    padding: 2px 6px;
    border-radius: 4px;
    font-size: 0.9em;
    color: var(--text-code);
}

.content-card-inner pre code {
    background: none;
    padding: 0;
    color: inherit;
}

.content-card-inner hr {
    border: none;
    border-top: 1px solid var(--border-medium);
    margin: 2em 0;
}

/* ========= 评论区 ========= */
.comments-section {
    margin-top: 48px;
}

.comments-divider {
    border: none;
    border-top: 1px solid var(--border-medium);
    margin: 0 0 32px 0;
}

.comments-title {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--text-heading);
    margin-bottom: 24px;
}

.comment-author {
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--text-comment-author);
    margin: 24px 0 12px;
}

.comment-author.anonymous {
    color: var(--text-comment-anonymous);
    font-weight: 500;
}

.comment-time {
    font-size: 0.85rem;
    font-weight: 400;
    color: var(--text-muted);
    margin-left: 8px;
}

/* ========= 上下篇导航 ========= */
.nav-buttons {
    display: flex;
    justify-content: space-between;
    align-items: center;
    gap: 16px;
    flex-wrap: wrap;
    margin: 48px 0 64px;
}

.nav-btn {
    background: var(--color-primary);
    border: none;
    padding: 12px 24px;
    border-radius: 40px;
    font-weight: 600;
    color: white;
    cursor: pointer;
    transition: all 0.2s;
    text-decoration: none;
    display: inline-flex;
    align-items: center;
    gap: 8px;
    font-size: 0.95rem;
}

.nav-btn:hover {
    opacity: 0.9;
    transform: translateY(-1px);
}

.nav-btn-disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.nav-btn-disabled:hover {
    opacity: 0.5;
    transform: none;
}

.btn-directory {
    background: var(--bg-card);
    color: var(--color-primary);
    border: 1px solid var(--border-nav);
}

.btn-directory:hover {
    background: var(--color-primary);
    color: white;
    border-color: var(--color-primary);
}

/* ========= 页脚 ========= */
footer {
    text-align: center;
    padding: 48px 24px 32px;
    color: var(--text-footer);
    border-top: 1px solid var(--border-footer);
    margin-top: 64px;
    font-size: 0.85rem;
}

.footer-links {
    margin-top: 8px;
    font-size: 0.8rem;
}

.footer-links a {
    color: var(--text-footer);
    text-decoration: underline;
    text-underline-offset: 3px;
    text-decoration-color: var(--border-footer);
    transition: color 0.2s;
}

.footer-links a:hover {
    color: var(--color-primary);
    text-decoration-color: var(--color-primary);
}

/* ========= 响应式 ========= */
@media (max-width: 640px) {
    .container {
        padding: 0 16px;
    }

    .nav-inner {
        flex-direction: column;
        gap: 8px;
        padding: 10px 16px;
    }

    .logo {
        font-size: 1.15rem;
    }

    .nav-links {
        gap: 12px;
        flex-wrap: wrap;
        justify-content: center;
        font-size: 0.85rem;
    }

    /* 移动端：下拉菜单使用点击展开 */
    .nav-dropdown {
        position: relative;
    }

    .nav-dropdown-toggle {
        pointer-events: auto;
        cursor: pointer;
        -webkit-tap-highlight-color: transparent;
    }

    .nav-dropdown-toggle .dropdown-arrow {
        display: inline-flex;
    }

    .nav-dropdown-menu {
        position: absolute;
        left: 50%;
        transform: translateX(-50%) translateY(-4px);
        top: 100%;
        opacity: 0;
        visibility: hidden;
        min-width: 160px;
        background: var(--bg-card);
        border: 1px solid var(--border-nav);
        border-radius: 12px;
        box-shadow: var(--shadow-hover);
        padding: 8px;
        margin-top: 8px;
        transition: opacity 0.2s, visibility 0.2s, transform 0.2s;
        display: block;
        z-index: 100;
    }

    .nav-dropdown-menu::before {
        display: block;
    }

    .nav-dropdown-menu.open {
        opacity: 1;
        visibility: visible;
        transform: translateX(-50%) translateY(0);
    }

    .nav-dropdown-menu a {
        display: block;
        padding: 8px 16px;
        font-size: 0.9rem;
        background: transparent;
        border-radius: 8px;
        white-space: nowrap;
    }

    .nav-dropdown-menu a:hover {
        background: var(--bg-hover);
        color: var(--color-primary);
    }

    .hero {
        padding: 32px 20px;
    }

    .hero h1 {
        font-size: 1.8rem;
    }

    .tab-btn {
        padding: 8px 16px;
        font-size: 0.9rem;
    }

    .lesson-item {
        flex-direction: column;
        align-items: flex-start;
        gap: 10px;
    }

    .content-card-inner {
        padding: 0 20px;
    }

    .nav-btn {
        padding: 8px 16px;
        font-size: 0.85rem;
    }
}

/* ========= 图片放大查看器 ========= */
.image-viewer {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1000;
    background: rgba(0, 0, 0, 0.9);
    cursor: default;
    user-select: none;
    -webkit-user-select: none;
}

.image-viewer.active {
    display: flex;
    align-items: center;
    justify-content: center;
}

.image-viewer-body {
    position: relative;
    width: 100%;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
}

.image-viewer-img {
    max-width: 90vw;
    max-height: 90vh;
    object-fit: contain;
    transition: transform 0.15s ease;
    border-radius: 4px;
    box-shadow: 0 4px 32px rgba(0,0,0,0.3);
    z-index: 10;
    cursor: grab;
    will-change: transform;
}

.image-viewer-img:active {
    cursor: grabbing;
}

/* 左右导航区域 */
.image-viewer-nav {
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    width: 48px;
    height: 48px;
    z-index: 20;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 50%;
    background: rgba(0,0,0,0.3);
    transition: background 0.2s, opacity 0.2s;
    cursor: default;
}

.image-viewer-nav:hover {
    background: rgba(0,0,0,0.6);
}

.image-viewer-nav--left {
    left: 16px;
}

.image-viewer-nav--right {
    right: 16px;
}

.image-viewer-nav svg {
    width: 28px;
    height: 28px;
    color: rgba(255,255,255,0.7);
    transition: color 0.2s;
    pointer-events: none;
}

.image-viewer-nav:hover svg {
    color: #fff;
}

.image-viewer-nav--hidden {
    display: none;
}

.image-viewer-close {
    position: absolute;
    top: 16px;
    right: 20px;
    background: rgba(255,255,255,0.15);
    border: none;
    color: #fff;
    font-size: 2rem;
    width: 44px;
    height: 44px;
    border-radius: 50%;
    cursor: pointer;
    z-index: 1010;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background 0.2s;
    line-height: 1;
}

.image-viewer-close:hover {
    background: rgba(255,255,255,0.3);
}

.image-viewer-counter {
    position: absolute;
    bottom: 20px;
    left: 50%;
    transform: translateX(-50%);
    color: rgba(255,255,255,0.7);
    font-size: 0.85rem;
    background: rgba(0,0,0,0.5);
    padding: 4px 14px;
    border-radius: 20px;
    z-index: 1010;
    pointer-events: none;
}

/* ========= 返回顶部按钮 ========= */
.back-to-top {
    position: fixed;
    bottom: 32px;
    right: 32px;
    width: 48px;
    height: 48px;
    background: var(--color-primary);
    color: white;
    border: none;
    border-radius: 50%;
    font-size: 1.5rem;
    cursor: pointer;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.3s, visibility 0.3s, transform 0.2s, background-color 0.3s;
    z-index: 100;
    display: grid;
    place-items: center;
    box-shadow: var(--shadow-btn);
    line-height: 1;
    padding: 0;
    margin: 0;
    -webkit-appearance: none;
}

.back-to-top.visible {
    opacity: 1;
    visibility: visible;
}

.back-to-top:hover {
    transform: translateY(-3px);
    background: var(--color-primary-hover);
    box-shadow: var(--shadow-btn-hover);
}

@media (max-width: 640px) {
    .back-to-top {
        bottom: 20px;
        right: 20px;
        width: 40px;
        height: 40px;
        font-size: 1.2rem;
    }
}