Avoid creating log files when no serial device is connected

src/serial_bridge.py

@@ -158,7 +158,6 @@ class SerialBridge(threading.Thread):
         self._log_file.write(f"{ts} {line}\n")
 
     def run(self) -> None:
-        self._open_log()
         try:
             while not self._stop_event.is_set():
                 if self._serial is None:
@@ -180,6 +179,7 @@ class SerialBridge(threading.Thread):
                 except Exception as exc:
                     self.broadcaster.publish(f"[bridge] disconnected: {exc}")
                     self._close_serial()
+                    self._close_log()
                     time.sleep(2)
         finally:
             self._close_serial()