feat: Implement background location tracking service

- Removed existing app icons from the asset catalog.
- Added new `Contents.json` for asset catalog.
- Updated `Info.plist` to include background modes and task scheduler identifiers.
- Exposed BLE service in `ConnectionProvider` for background tracking.
- Enhanced `MapTab` to support background location tracking with new service.
- Added `BackgroundLocationService` to manage location updates in the background.
- Updated `SettingsScreen` to allow users to configure GPS update distance and toggle background tracking.
- Implemented functionality to send location updates via BLE in `MeshCoreBleService`.
- Updated dependencies in `pubspec.yaml` for background service support.
- Adjusted `TileCacheService` to ensure ObjectBox is initialized only once.

lib/providers/connection_provider.dart

@@ -13,6 +13,9 @@ import '../utils/sar_message_parser.dart';
 class ConnectionProvider with ChangeNotifier {
   final MeshCoreBleService _bleService = MeshCoreBleService();
 
+  /// Expose BLE service for background location tracking
+  MeshCoreBleService get bleService => _bleService;
+
   DeviceInfo _deviceInfo = DeviceInfo();
   DeviceInfo get deviceInfo => _deviceInfo;