https://github.com/meshcore-dev/MeshCore/issues/989 - persist GPS enabled state to preferences

Add GPS configuration to NodePrefs structure and persist the GPS
enabled state when toggled via UI. This ensures GPS settings are
retained across device restarts.

examples/companion_radio/NodePrefs.h

@@ -25,4 +25,6 @@ struct NodePrefs {  // persisted to file
   uint32_t ble_pin;
   uint8_t  advert_loc_policy;
   uint8_t  buzzer_quiet;
+  uint8_t  gps_enabled;      // GPS enabled flag (0=disabled, 1=enabled)
+  uint32_t gps_interval;     // GPS read interval in seconds
 };