/* ========== 回到顶部按钮 ========== */
        .back-to-top {
            position: fixed;
            bottom: 40px;
            right: 40px;
            width: 50px;
            height: 50px;
            background: #667eea;
            color: white;
            border: none;
            border-radius: 50%;
            cursor: pointer;
            display: flex;
            align-items: center;
            justify-content: center;
            box-shadow: 0 4px 15px rgba(102, 126, 234, 0.4);
            opacity: 0;
            visibility: hidden;
            transform: translateY(20px);
            transition: all 0.3s ease;
            z-index: 999;
        }

        .back-to-top:hover {
            background: #764ba2;
            transform: translateY(-3px);
            box-shadow: 0 6px 20px rgba(102, 126, 234, 0.6);
        }

        .back-to-top.show {
            opacity: 1;
            visibility: visible;
            transform: translateY(0);
        }

        /* 箭头图标 */
        .back-to-top svg {
            width: 24px;
            height: 24px;
            fill: currentColor;
        }

        /* 滚动时的头部阴影变化（可选） */
        .header.scrolled {
            box-shadow: 0 4px 20px rgba(0,0,0,0.15);
        }