Add auth protection sample and standardize json.

frontend/src/app/components/HealthStatus.tsx

@@ -16,7 +16,14 @@ export default function HealthStatus() {
           throw new Error(`Server responded with status: ${response.status}`);
         }
 
-        const data = await response.json();
+        let data;
+        try {
+          data = await response.json();
+        } catch (jsonErr) {
+          setError("Response is not valid JSON");
+          setHealth({});
+          return;
+        }
         setHealth(data);
       } catch (err) {
         setError(

frontend/src/app/contexts/AuthContext.tsx

@@ -43,9 +43,24 @@ export function AuthProvider({ children }: { children: ReactNode }) {
       );
 
       if (response.ok) {
-        const userData = await response.json();
-        setUser(userData);
-        setIsAuthenticated(true);
+        let userData;
+        try {
+          userData = await response.json();
+        } catch (jsonErr) {
+          // 不是 JSON，降级处理
+          setUser(null);
+          setIsAuthenticated(false);
+          setLoading(false);
+          return;
+        }
+        // 处理后端返回 { error: ... }
+        if (userData && userData.error) {
+          setUser(null);
+          setIsAuthenticated(false);
+        } else {
+          setUser(userData);
+          setIsAuthenticated(true);
+        }
       } else {
         setUser(null);
         setIsAuthenticated(false);