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.

test/widget_test.dart

@@ -9,12 +9,17 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import 'package:family_safety_frontend/main.dart';
+import 'package:family_safety_frontend/services/background_geo_service.dart';
 import 'package:family_safety_frontend/services/settings_service.dart';
 
 void main() {
   testWidgets('App loads login screen', (WidgetTester tester) async {
+    final bgGeo = BackgroundGeoService();
+    await bgGeo.init();
+    await bgGeo.initNotifications();
     await tester.pumpWidget(MyApp(
       settingsService: SettingsService(),
+      bgGeo: bgGeo,
     ));
     await tester.pump();