Add Android background geolocation with notification error handling

Add flutter_background_geolocation for background location tracking on Android.
Service automatically sends coordinates to server when app is in background.
Error messages are shown as system notifications using flutter_local_notifications.

lib/screens/map_screen.dart

@@ -9,6 +9,7 @@ import '../providers/auth_provider.dart';
 import '../providers/map_provider.dart';
 import '../providers/share_provider.dart';
 import '../services/geo_service.dart';
+import '../services/background_geo_service.dart';
 
 class MapScreen extends StatefulWidget {
   const MapScreen({super.key});
@@ -69,6 +70,14 @@ class _MapScreenState extends State<MapScreen> {
             position.longitude,
             position.latitude,
           );
+
+          if (!mounted) return;
+          final bgGeo = context.read<BackgroundGeoService>();
+          bgGeo.configure(
+            token: authProvider.token,
+            geoId: shareProvider.geoId,
+          );
+          await bgGeo.start();
         } catch (e) {
           if (!mounted) return;
           ScaffoldMessenger.of(
@@ -219,8 +228,11 @@ class _MapScreenState extends State<MapScreen> {
           ),
           IconButton(
             icon: const Icon(Icons.logout),
-            onPressed: () {
+            onPressed: () async {
               _geoTimer?.cancel();
+              final bgGeo = context.read<BackgroundGeoService>();
+              await bgGeo.stop();
+              bgGeo.dispose();
               authProvider.logout();
               Navigator.of(context).pushReplacementNamed('/');
             },