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>

main.py

@@ -2,7 +2,7 @@ import time
 import signal
 import argparse
 
-from network_backend import NetworkBackend
+from backend.network import NetworkBackend
 from msg.frame_logger import FrameLogger
 from transceiver import Transceiver