fix: NAV头像session未同步导致不显示 + 图片加载失败回退首字符

- Validate中同步DB最新头像/用户名/角色到session，解决上传头像后NAV仍不显示的问题
- NAV模板img增加onerror回退，图片加载失败时自动显示用户名首字符

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

@@ -20,7 +20,8 @@
             <li class="nav-avatar-wrapper">
                 <a href="/space" class="nav-avatar-link">
                     {{if .Avatar}}
-                    <img src="{{.Avatar}}" alt="" class="nav-avatar">
+                    <img src="{{.Avatar}}" alt="" class="nav-avatar" onerror="this.style.display='none';this.nextElementSibling.style.display='flex'">
+                    <span class="nav-avatar nav-avatar-default" style="display:none">{{substr .Username 0 1}}</span>
                     {{else}}
                     <span class="nav-avatar nav-avatar-default">{{substr .Username 0 1}}</span>
                     {{end}}