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/services/geo_service.dart

@@ -7,7 +7,7 @@ class GeoService {
   
   GeoService({http.Client? client}) : _client = client ?? http.Client();
 
-  Future<void> updatePosition(String token, int id, double x, double y) async {
+  Future<void> updatePosition(String token, String id, double x, double y) async {
     final response = await _client.put(
       Uri.parse('${ApiConfig.geoUrl}?id=$id'),
       headers: {