/* --- 返回顶部按钮样式美化 --- */
.back-to-top {
    position: fixed;
    bottom: 30px;
    right: 30px;
    width: 50px;
    height: 50px;
    background-color: var(--card-bg);
    color: var(--primary-color);
    border-radius: 50%;
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
    border: none;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    opacity: 0;
    visibility: hidden;
    transform: translateY(20px);
    z-index: 999;
}

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

.back-to-top:hover {
    background-color: var(--primary-color);
    color: #fff;
    box-shadow: 0 6px 20px rgba(59, 130, 246, 0.4);
    transform: translateY(-3px);
}

.back-to-top svg {
    width: 24px;
    height: 24px;
    stroke-width: 2.5;
}