fix(ui): 修复空用户显示和 Vue 启动问题

- currentUser 初始值改为空对象，避免 null 访问错误
- template 中添加空值保护：currentUser.username ? ... : '?'
- 确保 Vue 应用能正常挂载

platform/frontend/topics.html

@@ -85,7 +85,7 @@
             <div class="navbar-content">
                 <h1 class="navbar-title">宇之然内容创作平台 - 选题管理</h1>
                 <div class="navbar-user">
-                    <div class="user-info"><div class="avatar">{{ currentUser.username.charAt(0).toUpperCase() }}</div><span>{{ currentUser.username }}</span></div>
+                    <div class="user-info"><div class="avatar">{{ currentUser.username ? currentUser.username.charAt(0).toUpperCase() : '?' }}</div><span>{{ currentUser.username }}</span></div>
                     <el-button type="danger" size="small" @click="handleLogout">退出</el-button>
                 </div>
             </div>
@@ -189,7 +189,7 @@ const TopicsApp = {
             currentPage: 'topics',
             isLoggedIn: false,
             isAdmin: false,
-            currentUser: null,
+            currentUser: { username:  },
             loadingTable: false,
             selectedTopicIds: [],
             filterStatus: '',