feat(frontend): prefill demo credentials on login page + E2E test passed

frontend/src/app/login/page.tsx

@@ -11,8 +11,8 @@ import { useAuth } from "@/lib/auth-context";
 export default function LoginPage() {
   const router = useRouter();
   const { login } = useAuth();
-  const [email, setEmail] = useState("");
-  const [password, setPassword] = useState("");
+  const [email, setEmail] = useState("gp@demo.com");
+  const [password, setPassword] = useState("demo123456");
   const [error, setError] = useState("");
   const [isLoading, setIsLoading] = useState(false);
 
@@ -102,6 +102,13 @@ export default function LoginPage() {
             {isLoading ? "登录中..." : "登录"}
           </button>
         </form>
+
+        {/* 演示账号提示 */}
+        <div className="mt-4 rounded-md border border-indigo-100 bg-indigo-50/50 p-3 text-xs text-muted-foreground">
+          <p className="font-medium text-indigo-600">演示账号</p>
+          <p className="mt-1">邮箱：gp@demo.com</p>
+          <p>密码：demo123456</p>
+        </div>
       </div>
     </div>
   );