nav.left-nav {
    background-color: #f1f1f1;
    width: 200px; /* 固定宽度 */
    padding: 10px;
    box-sizing: border-box;
    display: flex;
    flex-direction: column;
    overflow-y: auto; /* 添加滚动条 */
}
nav.left-nav a {
    display: block;
    width: 100%;
    height: 50px; /* 固定高度 */
    margin: 5px 0;
    text-decoration: none;
    color: #333;
    text-align: center;
    line-height: 50px; /* 垂直居中 */
    border-radius: 5px; /* 圆角 */
    transition: background-color 0.3s; /* 平滑过渡 */
    background-color: #ddd;
}
nav.left-nav a.selected {
    font-weight: bold;
    font-size: 18px;
    background-color: #ccc;
}
nav.left-nav a:hover {
    background-color: #bbb; /* 鼠标移上去时变色 */
    /* font-weight: bold; */
}

/* 响应式设计 */
@media (max-width: 768px) {
    nav.left-nav {
        display: none;
    }
}