[RAPID-micro] IMAP 연결 테스트 실패 시 상세 에러 메시지 표시

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

backend-node/src/services/userMailImapService.ts

@@ -390,7 +390,13 @@ class UserMailImapService {
       await client.logout();
       return { success: true, message: 'IMAP 연결 성공' };
     } catch (err) {
-      return { success: false, message: err instanceof Error ? err.message : '연결 실패' };
+      let message = '연결 실패';
+      if (err instanceof Error) {
+        const imapErr = err as any;
+        const detail = imapErr.response || imapErr.responseCode || imapErr.cause?.message;
+        message = detail ? `${err.message}: ${detail}` : err.message;
+      }
+      return { success: false, message };
     }
   }
 }