Add toggle to disable printer firmware update checks

Add new setting "Check printer firmware" in Settings → General → Updates
that allows users to disable automatic firmware update checks from
Bambu Lab servers.

Closes #169

backend/app/api/routes/settings.py

@@ -74,6 +74,7 @@ async def get_settings(db: AsyncSession = Depends(get_db)):
                 "capture_finish_photo",
                 "spoolman_enabled",
                 "check_updates",
+                "check_printer_firmware",
                 "virtual_printer_enabled",
                 "ftp_retry_enabled",
                 "mqtt_enabled",