fix: Google OAuth 로그인 시 localhost:3000 리다이렉트 문제 수정

- app/auth/callback/route.ts: NEXT_PUBLIC_SITE_URL 우선, x-forwarded-host 폴백
  Vercel 내부 라우팅에서 request.url의 origin이 localhost로 잡히는 경우 대응
- app/login/page.tsx: signInWithOAuth redirectTo를 NEXT_PUBLIC_SITE_URL 기준으로 변경
  window.location.origin 대신 명시적 환경변수 사용으로 안정성 확보

Vercel 환경변수에 NEXT_PUBLIC_SITE_URL=https://<도메인> 추가 필요

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

app/auth/callback/route.ts

@@ -6,21 +6,23 @@ export async function GET(request: Request) {
   const code = searchParams.get('code');
   const next = searchParams.get('next') ?? '/mypage';
 
+  // 프로덕션 기준 URL 결정
+  // 우선순위: NEXT_PUBLIC_SITE_URL > x-forwarded-host > origin
+  const siteUrl = process.env.NEXT_PUBLIC_SITE_URL;
+  const forwardedHost = request.headers.get('x-forwarded-host');
+  const baseUrl =
+    siteUrl ??
+    (process.env.NODE_ENV !== 'development' && forwardedHost
+      ? `https://${forwardedHost}`
+      : origin);
+
   if (code) {
     const supabase = await createClient();
     const { error } = await supabase.auth.exchangeCodeForSession(code);
     if (!error) {
-      const forwardedHost = request.headers.get('x-forwarded-host');
-      const isLocalEnv = process.env.NODE_ENV === 'development';
-      if (isLocalEnv) {
-        return NextResponse.redirect(`${origin}${next}`);
-      } else if (forwardedHost) {
-        return NextResponse.redirect(`https://${forwardedHost}${next}`);
-      } else {
-        return NextResponse.redirect(`${origin}${next}`);
-      }
+      return NextResponse.redirect(`${baseUrl}${next}`);
     }
   }
 
-  return NextResponse.redirect(`${origin}/login?error=auth-callback-error`);
+  return NextResponse.redirect(`${baseUrl}/login?error=auth-callback-error`);
 }