backend: move gui/backends.py to backend/serial.py

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

gui/data_model.py

@@ -147,7 +147,7 @@ class ReceiverManager(QObject):
 
     def connect_receiver(self, rid: str):
         from listeners import NavSatQtListener, GsaQtListener
-        from backends  import SerialBackend
+        from backend.serial import SerialBackend
 
         config = self._configs.get(rid)
         state  = self._states.get(rid)