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/html/layout/nav.html

@@ -27,9 +27,34 @@
                     {{end}}
                 </a>
                 <div class="nav-dropdown">
-                    <div class="nav-dropdown-header">{{.Username}}</div>
-                    <a href="/settings/profile" class="nav-dropdown-item">个人中心</a>
-                    <a href="javascript:void(0)" class="nav-dropdown-item nav-logout" id="logoutBtn">退出登录</a>
+                    <div class="nav-dropdown-header">
+                        <div class="nav-dropdown-avatar">
+                            {{if .Avatar}}
+                            <img src="{{.Avatar}}" alt="" onerror="this.style.display='none';this.nextElementSibling.style.display='flex'">
+                            <span style="display:none">{{substr .Username 0 1}}</span>
+                            {{else}}
+                            <span>{{substr .Username 0 1}}</span>
+                            {{end}}
+                        </div>
+                        <div class="nav-dropdown-meta">
+                            <div class="nav-dropdown-username">{{.Username}}</div>
+                            {{if .IsLoggedIn}}
+                            <div class="nav-dropdown-level">Lv{{.Level}}</div>
+                            {{end}}
+                        </div>
+                    </div>
+                    <a href="javascript:void(0)" class="nav-dropdown-item" id="checkInBtn">
+                        <svg class="nav-dropdown-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="20 6 9 17 4 12"/></svg>
+                        <span>签到</span>
+                    </a>
+                    <a href="/settings/profile" class="nav-dropdown-item">
+                        <svg class="nav-dropdown-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M20 21v-2a4 4 0 0 0-4-4H8a4 4 0 0 0-4 4v2"/><circle cx="12" cy="7" r="4"/></svg>
+                        <span>个人中心</span>
+                    </a>
+                    <a href="javascript:void(0)" class="nav-dropdown-item nav-logout" id="logoutBtn">
+                        <svg class="nav-dropdown-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4"/><polyline points="16 17 21 12 16 7"/><line x1="21" y1="12" x2="9" y2="12"/></svg>
+                        <span>退出登录</span>
+                    </a>
                 </div>
             </li>
             {{else}}