Replace paid flutter_background_geolocation with free flutter_foreground_task

Replace proprietary flutter_background_geolocation (requires paid license
for Android release builds) with free flutter_foreground_task package.
Background location tracking now uses foreground service with periodic
geolocation updates every 30 seconds.

lib/screens/map_screen.dart

@@ -10,6 +10,7 @@ import '../providers/map_provider.dart';
 import '../providers/share_provider.dart';
 import '../services/geo_service.dart';
 import '../services/background_geo_service.dart';
+import 'package:flutter_foreground_task/flutter_foreground_task.dart';
 
 class MapScreen extends StatefulWidget {
   const MapScreen({super.key});
@@ -77,6 +78,7 @@ class _MapScreenState extends State<MapScreen> {
             token: authProvider.token,
             geoId: shareProvider.geoId,
           );
+          FlutterForegroundTask.setTaskHandler(BackgroundTaskHandler());
           await bgGeo.start();
         } catch (e) {
           if (!mounted) return;