Fix late background alerts and cache starvation

- Increase background refresh threshold: Raised the edge distance check in GpsController from 1000m to 2500m to guarantee new Overpass data is fetched well before the user drives out of the cached bounding box.

- Fix bounding box race condition: Added await to the background fetch handler in MapView so tracking bounds only shift forward after the network request successfully finishes.

- Make debounced fetches awaitable: Refactored fetchAndUpdate in NodeProviderWithCache to return a Future<void> using a Completer, bridging the 400ms debounce timer.

- Prevent hanging futures: Added state checks for _activeCompleter to instantly resolve canceled network requests during rapid map panning, preventing fatal StateError crashes and infinite UI hangs.

lib/widgets/map/gps_controller.dart

@@ -333,9 +333,9 @@ class GpsController {
       final minDist = [northDist, southDist, eastDist, westDist].reduce((a, b) => a < b ? a : b);
 
       // Debug 2: Print the closest distance to the edge
-      debugPrint('[GpsController-Background] Closest edge distance: ${minDist.toStringAsFixed(1)}m (Threshold: 1000m)');
+      debugPrint('[GpsController-Background] Closest edge distance: ${minDist.toStringAsFixed(1)}m (Threshold: 2500m)');
 
-      if (minDist < 1000) {
+      if (minDist < 2500) {
         debugPrint('[GpsController-Background] Threshold crossed. Triggering refresh.');
         needsRefresh = true;
       }
@@ -554,4 +554,4 @@ class GpsController {
     _onMapMovedProgrammatically = null;
     _isUserInteracting = null;
   }
-}
+}