feat: 实现积分与等级系统 + 签到功能 + 下拉菜单优化

- User 模型增加 exp 字段，等级规则 Lv0-Lv6
- 新增签到记录(UserCheckIn)与任务记录(UserTask)模型，唯一索引防重
- 自动签到：AuthMiddleware 验证会话后，基于 Asia/Shanghai 自然日自动签到 +5 经验
- 手动签到：下拉菜单"签到"按钮兜底，已签到显示"今日已签到"并禁用
- 首次任务经验：上传头像/设置用户名/设置个性签名各 +10，审核通过与直接更新均触发
- 经验值变动后自动比对等级，升级时通过通知系统发送"恭喜提升至 LvX"
- 下拉菜单重构：顶部头像+用户名+LvX 等级标签，菜单项增加左侧图标，优化布局样式
- 新增 API：POST /api/level/checkin、GET /api/level/info

templates/MetaLab-2026/static/css/common.css

@@ -262,17 +262,65 @@ header {
 }
 
 .nav-dropdown-header {
-    padding: .75rem 1rem;
-    font-weight: 600;
-    font-size: .9rem;
-    color: var(--color-text);
+    display: flex;
+    align-items: center;
+    gap: .75rem;
+    padding: 1rem;
     border-bottom: 1px solid var(--color-border);
+}
+
+.nav-dropdown-avatar {
+    width: 48px;
+    height: 48px;
+    border-radius: 50%;
+    overflow: hidden;
+    flex-shrink: 0;
+    background: var(--color-accent);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    color: #fff;
+    font-size: 1.1rem;
+    font-weight: 600;
+}
+
+.nav-dropdown-avatar img {
+    width: 100%;
+    height: 100%;
+    object-fit: cover;
+}
+
+.nav-dropdown-meta {
+    min-width: 0;
+    overflow: hidden;
+}
+
+.nav-dropdown-username {
+    font-weight: 600;
+    font-size: .95rem;
+    color: var(--color-text);
     white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
+.nav-dropdown-level {
+    display: inline-block;
+    margin-top: .2rem;
+    padding: .1rem .5rem;
+    border-radius: 4px;
+    background: linear-gradient(135deg, #ff6b6b, #feca57);
+    color: #fff;
+    font-size: .75rem;
+    font-weight: 700;
+    line-height: 1.4;
 }
 
 .nav-dropdown-item {
-    display: block !important;
-    padding: .6rem 1rem !important;
+    display: flex !important;
+    align-items: center;
+    gap: .6rem;
+    padding: .65rem 1rem !important;
     font-size: .9rem;
     color: var(--color-text-light) !important;
     transition: background .15s ease, color .15s ease;
@@ -291,6 +339,28 @@ header {
     color: var(--color-danger) !important;
 }
 
+.nav-dropdown-icon {
+    width: 18px;
+    height: 18px;
+    flex-shrink: 0;
+    opacity: .7;
+}
+
+.nav-dropdown-item:hover .nav-dropdown-icon {
+    opacity: 1;
+}
+
+.nav-dropdown-item.disabled {
+    opacity: .5;
+    cursor: not-allowed;
+    pointer-events: none;
+}
+
+.nav-dropdown-item.disabled:hover {
+    background: transparent;
+    color: var(--color-text-light) !important;
+}
+
 .mobile-menu-btn {
     display: none;
     background: none;