/* 前台导航栏 */
.navbar {
    background: #fff;
    padding: 15px 50px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    box-shadow: 0 2px 10px rgba(0,0,0,0.05);
}

.logo { font-size: 24px; font-weight: bold; color: #3498db; text-decoration: none; }

.user-nav a {
    margin-left: 20px;
    text-decoration: none;
    color: #666;
    font-size: 14px;
}

/* 首页大横幅 */
.hero {
    text-align: center;
    padding: 60px 20px;
    background: linear-gradient(135deg, #74b9ff, #a29bfe);
    color: white;
}

.hero h1 { font-size: 36px; margin-bottom: 10px; }

/* 选项卡切换 */
.tabs {
    display: flex;
    justify-content: center;
    margin: 30px 0;
    gap: 20px;
}

.tab-btn {
    padding: 12px 30px;
    border-radius: 30px;
    border: 2px solid #3498db;
    background: transparent;
    color: #3498db;
    cursor: pointer;
    font-size: 16px;
    font-weight: bold;
    transition: 0.3s;
}

.tab-btn.active {
    background: #3498db;
    color: white;
}

/* 练习卡片列表 */
.article-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 25px;
    padding: 0 50px 50px;
    max-width: 1200px;
    margin: 0 auto;
}

.article-card {
    background: white;
    padding: 25px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.05);
    transition: transform 0.3s;
    text-align: center;
    border: 1px solid #eee;
}

.article-card:hover {
    transform: translateY(-5px);
    border-color: #3498db;
}

.article-card h3 { margin-bottom: 15px; color: #333; font-size: 18px; }

.btn-start {
    display: inline-block;
    padding: 10px 25px;
    background: #2ecc71;
    color: white;
    text-decoration: none;
    border-radius: 20px;
    font-weight: bold;
}