Hide default authorized email in login UI

frontend/src/app/login/LoginForm.tsx

@@ -8,7 +8,7 @@ import { loginWithEmail, normalizeAuthError } from "../../lib/auth";
 export function LoginForm() {
   const router = useRouter();
   const searchParams = useSearchParams();
-  const [email, setEmail] = useState("young@devease.ai");
+  const [email, setEmail] = useState("");
   const [accessCode, setAccessCode] = useState("");
   const [message, setMessage] = useState<string | null>(null);
   const [isSubmitting, setIsSubmitting] = useState(false);
@@ -37,7 +37,7 @@ export function LoginForm() {
         inputMode="email"
         name="email"
         onChange={(event) => setEmail(event.target.value)}
-        placeholder="young@devease.ai"
+        placeholder="请输入授权邮箱"
         required
         type="email"
         value={email}

frontend/src/app/login/page.test.tsx

@@ -18,7 +18,7 @@ describe("login page", () => {
     expect(html).toContain("Beatbot");
     expect(html).toContain("授权邮箱");
     expect(html).toContain("访问码");
-    expect(html).toContain("young@devease.ai");
+    expect(html).not.toContain("young@devease.ai");
     expect(html).toContain("登录");
   });
 });