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/auth_service.dart

@@ -4,7 +4,7 @@ import '../config/api.dart';
 
 class AuthService {
   final http.Client _client;
-  
+
   AuthService({http.Client? client}) : _client = client ?? http.Client();
 
   Future<String> login(String login, String password) async {
@@ -13,7 +13,7 @@ class AuthService {
       headers: {'Content-Type': 'application/json'},
       body: jsonEncode({'login': login, 'password': password}),
     );
-    
+
     if (response.statusCode == 200) {
       return response.body;
     } else {
@@ -21,15 +21,23 @@ class AuthService {
     }
   }
 
-  Future<void> register(String login, String password) async {
+  Future<void> register(
+    String login,
+    String password,
+    String secretKeyHash,
+  ) async {
     final response = await _client.post(
       Uri.parse(ApiConfig.regUrl),
       headers: {'Content-Type': 'application/json'},
-      body: jsonEncode({'login': login, 'password': password}),
+      body: jsonEncode({
+        'login': login,
+        'password': password,
+        'secret_key_hash': secretKeyHash,
+      }),
     );
-    
+
     if (response.statusCode != 201) {
       throw Exception('Registration failed');
     }
   }
-}
+}