feat: LoginPage DEV_LOGIN 환경변수 지원 추가

VITE_ENABLE_DEV_LOGIN=true로 프로덕션 빌드에서도 DEV LOGIN 활성화 가능.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/auth/LoginPage.tsx

@@ -8,6 +8,7 @@ interface LoginPageProps {
 
 const GOOGLE_CLIENT_ID = import.meta.env.VITE_GOOGLE_CLIENT_ID;
 const IS_DEV = import.meta.env.DEV;
+const DEV_LOGIN_ENABLED = IS_DEV || import.meta.env.VITE_ENABLE_DEV_LOGIN === 'true';
 
 function useGoogleIdentity(onCredential: (credential: string) => void) {
   const btnRef = useRef<HTMLDivElement>(null);
@@ -136,7 +137,7 @@ const LoginPage = ({ onGoogleLogin, onDevLogin }: LoginPageProps) => {
         )}
 
         {/* Dev Login */}
-        {IS_DEV && (
+        {DEV_LOGIN_ENABLED && (
           <>
             <div className="w-full border-t border-kcg-border pt-4 text-center">
               <span className="text-xs font-mono tracking-wider text-kcg-dim">