feat: 封禁用户 NAV 横幅 + Space 页面隐藏个性签名

- BuildPageData 注入 IsBanned 字段，封禁用户所有页面显示红色横幅
- Space 页面对封禁用户覆写 Bio 为'该账号已被封禁'，原文保留
- 新增 banned-banner 样式（浅红底深红字）

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

@@ -73,3 +73,11 @@
     {{end}}
 </div>
 {{end}}
+{{if .IsBanned}}
+<div class="maintenance-banner banned-banner">
+    <span class="maintenance-banner-icon">
+        <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="10"/><line x1="12" y1="16" x2="12" y2="12"/><line x1="12" y1="8" x2="12.01" y2="8"/></svg>
+    </span>
+    <span>该账号已被封禁</span>
+</div>
+{{end}}

templates/MetaLab-2026/html/space/index.html

@@ -24,7 +24,7 @@
             </div>
             <div class="space-info">
                 <h1 class="space-username">{{.SpaceUser.Username}}</h1>
-                <p class="space-bio">{{if .SpaceUser.Bio}}{{.SpaceUser.Bio}}{{else}}&nbsp;{{end}}</p>
+                <p class="space-bio">{{if eq .SpaceUser.Status "banned"}}<svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="vertical-align:-3px;margin-right:4px"><circle cx="12" cy="12" r="10"/><line x1="12" y1="16" x2="12" y2="12"/><line x1="12" y1="8" x2="12.01" y2="8"/></svg>该账号已被封禁{{else if .SpaceUser.Bio}}{{.SpaceUser.Bio}}{{else}}&nbsp;{{end}}</p>
                 <div class="space-meta">
                     <span class="space-uid">UID: {{.SpaceUser.ID}}</span>
                     <span class="space-divider">·</span>