fix: 修复 hooks.ts 中 username/role 可能 undefined 的问题

- 使用 nullish coalescing operator 处理可能的 undefined 值
- 确保 localStorage 存储的值始终有效

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend-v2/lib/hooks.ts

@@ -54,14 +54,16 @@ export function useAuth() {
       if (response.session_id) {
         console.log('[useAuth] Login successful, setting state...');
         setIsAuthenticated(true);
-        setUsername(response.username);
-        setRole(response.role);
+        const username = response.username ?? 'Unknown';
+        const role = response.role ?? 'user';
+        setUsername(username);
+        setRole(role);
         // 持久化到 localStorage
         localStorage.setItem('session_id', response.session_id);
-        localStorage.setItem('username', response.username);
+        localStorage.setItem('username', username);
         const userInfo: UserInfo = {
-          username: response.username,
-          role: response.role,
+          username,
+          role,
         };
         localStorage.setItem('userInfo', JSON.stringify(userInfo));
         console.log('[useAuth] State updated, userInfo:', userInfo);
@@ -85,14 +87,16 @@ export function useAuth() {
       const loginResponse = await apiClient.login(email, password);
       if (loginResponse.session_id) {
         setIsAuthenticated(true);
-        setUsername(loginResponse.username);
-        setRole(loginResponse.role);
+        const loginUsername = loginResponse.username ?? username;
+        const loginRole = loginResponse.role ?? 'user';
+        setUsername(loginUsername);
+        setRole(loginRole);
         // 持久化到 localStorage
         localStorage.setItem('session_id', loginResponse.session_id);
-        localStorage.setItem('username', loginResponse.username);
+        localStorage.setItem('username', loginUsername);
         const userInfo: UserInfo = {
-          username: loginResponse.username,
-          role: loginResponse.role,
+          username: loginUsername,
+          role: loginRole,
         };
         localStorage.setItem('userInfo', JSON.stringify(userInfo));
         return { ...registerResponse, session: loginResponse };