harden UBX parsing/dispatch and report peer-side disconnects in GUI

A struct size-mismatch or any other parse error in a listener could
propagate uncaught out of the socket-reading thread and silently kill
it. Catch broadly in the event loop and per-listener in call_listener,
and make the NAV-SAT/NAV-SIG/RXM-RAWX group parsers tolerate a
payload length that isn't an exact multiple of the group size instead
of crashing on a short struct.unpack.

Also wire NetworkBackend's socket-loss detection through to
ReceiverManager (via a queued signal, since the callback fires from
the backend thread) so the GUI reflects a peer-initiated disconnect
instead of leaving the row stuck showing "connected". Verified against
a real ZED-X20P: it drops the TCP session on its own after ~20-28s
regardless of traffic; the GUI now marks the receiver disconnected and
frees its ID for reconnecting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AzmCaNjDb3TAqPpTwunKTY

backend/network_backend.py

@@ -10,11 +10,12 @@ from backend.a_backend import ABackend
 
 
 class NetworkBackend(ABackend):
-	def __init__(self, servers: list[dict]):
+	def __init__(self, servers: list[dict], on_disconnect=None):
 		self.sel = None
 		self.sock_list = [{'name': s['name'], 'addr': (socket.gethostbyname(s['host']), s['port']), 'xcvr': None, 'queue': Queue()} for s in servers]
 		self.thread = Thread(target=self.event_loop)
 		self.loop_enable = True
+		self.on_disconnect = on_disconnect
 
 	def send(self, xcvr: ATransceiver, data: bytes):
 		sock = self.find_by_name(xcvr.name)
@@ -89,6 +90,10 @@ class NetworkBackend(ABackend):
 						print(f"Connection {sock['name']} to {sock['addr']} lost: {exc}")
 						self.sel.unregister(key.fileobj)
 						key.fileobj.close()
+						if self.on_disconnect:
+							self.on_disconnect(sock['name'])
+					except Exception as exc:
+						print(f"Connection {sock['name']}: error processing data, ignoring: {exc}")
 
 		except KeyboardInterrupt:
 			print("Caught keyboard interrupt, exiting")