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/providers/auth_provider.dart

@@ -5,24 +5,24 @@ import '../services/auth_service.dart';
 
 class AuthProvider with ChangeNotifier {
   final AuthService _authService;
-  
+
   String _token = '';
   bool _isLoading = false;
   String _error = '';
-  
-  AuthProvider({AuthService? authService}) 
-      : _authService = authService ?? AuthService();
-  
+
+  AuthProvider({AuthService? authService})
+    : _authService = authService ?? AuthService();
+
   String get token => _token;
   bool get isLoggedIn => _token.isNotEmpty;
   bool get isLoading => _isLoading;
   String get error => _error;
-  
+
   Future<void> login(String login, String password) async {
     _isLoading = true;
     _error = '';
     notifyListeners();
-    
+
     try {
       var response = await _authService.login(login, password);
       var data = jsonDecode(response);
@@ -37,14 +37,18 @@ class AuthProvider with ChangeNotifier {
       notifyListeners();
     }
   }
-  
-  Future<void> register(String login, String password) async {
+
+  Future<void> register(
+    String login,
+    String password,
+    String secretKeyHash,
+  ) async {
     _isLoading = true;
     _error = '';
     notifyListeners();
-    
+
     try {
-      await _authService.register(login, password);
+      await _authService.register(login, password, secretKeyHash);
       notifyListeners();
     } catch (e) {
       _error = e.toString();
@@ -55,9 +59,14 @@ class AuthProvider with ChangeNotifier {
       notifyListeners();
     }
   }
-  
+
+  void setError(String message) {
+    _error = message;
+    notifyListeners();
+  }
+
   void logout() {
     _token = '';
     notifyListeners();
   }
-}
+}