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/settings_service.dart

@@ -0,0 +1,28 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+class SettingsService {
+  static const String _baseUrlKey = 'server_base_url';
+  static const String _defaultBaseUrl = 'https://family-safety.onrender.com/api';
+
+  String _baseUrl = _defaultBaseUrl;
+  bool _initialized = false;
+
+  String get baseUrl => _baseUrl;
+
+  Future<void> initialize() async {
+    if (_initialized) return;
+    final prefs = await SharedPreferences.getInstance();
+    _baseUrl = prefs.getString(_baseUrlKey) ?? _defaultBaseUrl;
+    _initialized = true;
+  }
+
+  Future<void> setBaseUrl(String url) async {
+    _baseUrl = url;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_baseUrlKey, url);
+  }
+
+  void resetToDefault() {
+    _baseUrl = _defaultBaseUrl;
+  }
+}