Add Android support with configurable server URL and location permissions

- Add shared_preferences for persisting server URL
- Add SettingsService and PlatformService
- Add server URL input field on non-web platforms
- Make ApiConfig baseUrl configurable at runtime
- Add Android location permissions (ACCESS_FINE/COURSE_LOCATION, INTERNET)
- Request location permission on login and map init
- Fix geo_id type: use String instead of int (UUID format)
- Align share_service with API spec: remove unique_id, use share_id only
- Fix watch endpoint response: last_update instead of created_at
- Add error handling with SnackBars for geo operations
- Wrap login screen in SingleChildScrollView for keyboard handling
- Update map tile layer with userAgentPackageName for OSM

lib/main.dart

@@ -1,22 +1,31 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
+import 'config/api.dart';
 import 'providers/auth_provider.dart';
 import 'providers/map_provider.dart';
 import 'providers/share_provider.dart';
 import 'screens/login_screen.dart';
 import 'screens/map_screen.dart';
+import 'services/settings_service.dart';
 
-void main() {
-  runApp(const MyApp());
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  final settings = SettingsService();
+  await settings.initialize();
+  ApiConfig.setBaseUrl(settings.baseUrl);
+  runApp(MyApp(settingsService: settings));
 }
 
 class MyApp extends StatelessWidget {
-  const MyApp({super.key});
+  final SettingsService settingsService;
+
+  const MyApp({super.key, required this.settingsService});
 
   @override
   Widget build(BuildContext context) {
     return MultiProvider(
       providers: [
+        Provider.value(value: settingsService),
         ChangeNotifierProvider(create: (_) => AuthProvider()),
         ChangeNotifierProvider(create: (_) => MapProvider()),
         ChangeNotifierProvider(create: (_) => ShareProvider()),
@@ -37,4 +46,4 @@ class MyApp extends StatelessWidget {
       ),
     );
   }
-}
+}