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.

test/widget_test.dart

@@ -16,7 +16,6 @@ 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,