fix: CSP 策略移除 unsafe-inline，改用 nonce 机制

- SecurityHeaders 中间件生成随机 nonce，注入 context
- BuildPageData/BuildAdminPageData 将 CSPNonce 传递给模板
- 所有 19 个模板文件 <script> 标签添加 nonce 属性
- 移除所有内联事件处理（onclick/onerror），改用事件监听
- 移除所有 javascript:void(0) 链接，改用 # 号 + preventDefault
- utils.js 添加全局 avatar 图片加载失败回退处理
- common.js 登出按钮添加 preventDefault
- audit.js 关闭弹窗按钮改用事件监听

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

@@ -20,7 +20,7 @@
             <li class="nav-avatar-wrapper">
                 <a href="/space" class="nav-avatar-link">
                     {{if .Avatar}}
-                    <img src="{{.Avatar}}" alt="" class="nav-avatar" onerror="this.style.display='none';this.nextElementSibling.style.display='flex'">
+                    <img src="{{.Avatar}}" alt="" class="nav-avatar">
                     <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>
@@ -30,7 +30,7 @@
                     <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'">
+                            <img src="{{.Avatar}}" alt="">
                             <span style="display:none">{{substr .Username 0 1}}</span>
                             {{else}}
                             <span>{{substr .Username 0 1}}</span>
@@ -47,7 +47,7 @@
                         <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">
+                    <a href="#" 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>