Fix SftpService to access SftpException ID directly via the 'id' field for improved error message formatting.

backend/src/main/java/com/sshmanager/service/SftpService.java

@@ -120,7 +120,7 @@ public class SftpService {
      * Build a user-visible message from JSch SftpException (getMessage() is often null).
      */
     public static String formatSftpExceptionMessage(SftpException e, String path, String operation) {
-        int id = e.getId();
+        int id = e.id;
         String serverMsg = e.getMessage();
         String reason = sftpErrorCodeToMessage(id);
         StringBuilder sb = new StringBuilder();