refactor: 重写导航栏布局与交互

- 导航项重新排序：首页、投稿、信封图标、头像下拉菜单
- 铃铛图标替换为信封图标
- 用户名/设置/退出移入头像 hover 下拉菜单
- 设置改名为个人中心，退出改名为退出登录
- 头像点击跳转 /space，hover 展开下拉菜单
- 无头像时显示用户名首字彩色圆形占位
- Avatar 数据链路打通：Session→Middleware→BuildPageData→模板
- FindByIDForAuth 查询增加 avatar 列
- 新增 substr 模板函数

templates/MetaLab-2026/html/layout/nav.html

@@ -6,21 +6,31 @@
         </button>
         <ul class="nav-links" id="navLinks">
             <li><a href="/">首页</a></li>
-            <li><a href="/posts">帖子</a></li>
             {{if .IsLoggedIn}}
             <li><a href="/studio/write" class="nav-contribute">
                 <svg class="icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M12 20h9"/><path d="M16.5 3.5a2.121 2.121 0 0 1 3 3L7 19l-4 1 1-4L16.5 3.5z"/></svg>
                 投稿
             </a></li>
             <li>
-                <a href="/messages" class="nav-messages" id="messagesBell" title="消息中心">
-                    <svg class="nav-bell-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M18 8A6 6 0 0 0 6 8c0 7-3 9-3 9h18s-3-2-3-9"/><path d="M13.73 21a2 2 0 0 1-3.46 0"/></svg>
-                    <span class="nav-bell-badge" id="msgBadge" style="display:none">0</span>
+                <a href="/messages" class="nav-envelope" id="messagesEnvelope" title="消息中心">
+                    <svg class="nav-envelope-icon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M4 4h16c1.1 0 2 .9 2 2v12c0 1.1-.9 2-2 2H4c-1.1 0-2-.9-2-2V6c0-1.1.9-2 2-2z"/><polyline points="22,6 12,13 2,6"/></svg>
+                    <span class="nav-envelope-badge" id="msgBadge" style="display:none">0</span>
                 </a>
             </li>
-            <li><a href="/space" class="nav-user">{{.Username}}</a></li>
-            <li><a href="/settings/profile" class="nav-settings">设置</a></li>
-            <li><a href="javascript:void(0)" class="nav-logout" id="logoutBtn">退出</a></li>
+            <li class="nav-avatar-wrapper">
+                <a href="/space" class="nav-avatar-link">
+                    {{if .Avatar}}
+                    <img src="{{.Avatar}}" alt="" class="nav-avatar">
+                    {{else}}
+                    <span class="nav-avatar nav-avatar-default">{{substr .Username 0 1}}</span>
+                    {{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>
+            </li>
             {{else}}
             <li><a href="/auth/login" class="nav-login">登录</a></li>
             {{end}}