fix(ui): 修复 Vue 应用初始化语法错误

- currentUser 初始化改为 { username: '' }（之前遗漏空字符串）
- 通过 node --check 验证语法正确

platform/frontend/topics.html

@@ -189,7 +189,7 @@ const TopicsApp = {
             currentPage: 'topics',
             isLoggedIn: false,
             isAdmin: false,
-            currentUser: { username:  },
+            currentUser: { username: '' },
             loadingTable: false,
             selectedTopicIds: [],
             filterStatus: '',