backend: move NetworkBackend and ABackend into backend/ folder

- network_backend.py -> backend/network.py
- a_backend.py -> backend/a_backend.py
- update all imports accordingly

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

gui/data_model.py

@@ -11,7 +11,7 @@ for _p in [_root, _gui]:
 from transceiver import Transceiver
 from nmea.packetizer import NmeaPacketizer
 from ubx.packetizer import UbxPacketizer
-from network_backend import NetworkBackend
+from backend.network import NetworkBackend
 from ubx.ubx import frame_create
 
 GNSS_NAMES = {