fix: 로그인 페이지 하드코딩된 테스트 계정 제거

- userId/password 초기값을 빈 문자열로 변경
- 개발용 TestUser5/password123! 제거

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

src/components/auth/LoginPage.tsx

@@ -24,8 +24,8 @@ export function LoginPage() {
   const t = useTranslations('auth');
   const tCommon = useTranslations('common');
   const tValidation = useTranslations('validation');
-  const [userId, setUserId] = useState("TestUser5");
-  const [password, setPassword] = useState("password123!");
+  const [userId, setUserId] = useState("");
+  const [password, setPassword] = useState("");
   const [showPassword, setShowPassword] = useState(false);
   const [rememberMe, setRememberMe] = useState(false);
   const [error, setError] = useState("");