feat(03-02): integrate OverlayService into OverlayProvider with error callbacks and GPS grace period

- Add OverlayError enum (permission, creationFailed, communicationDegraded)
- Add OverlayService dependency via constructor injection
- Add onError callback field for error surfacing (set by SpeedometerScreen)
- Refactor showOverlay to check permission before showing overlay
- Refactor showOverlay to verify overlay creation via isActive check
- Only set _isOverlayActive after verification succeeds
- Replace all FlutterOverlayWindow calls with OverlayService methods
- Add GPS grace period timer (30s) in _handleOverlayClose
- Cancel grace period if overlay reopens within 30s
- Register OverlayService in MultiProvider before OverlayProvider
- Inject OverlayService into OverlayProvider constructor

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/main.dart

@@ -4,6 +4,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'screens/speedometer_screen.dart';
 import 'screens/overlay_screen.dart';
 import 'services/gps_data_manager.dart';
+import 'services/overlay_service.dart';
 import 'providers/settings_provider.dart';
 import 'providers/overlay_provider.dart';
 import 'services/logger.dart';
@@ -16,6 +17,7 @@ void main() async {
     MultiProvider(
       providers: [
         Provider<SharedPreferences>.value(value: prefs),
+        Provider<OverlayService>(create: (_) => OverlayService()),
         ChangeNotifierProvider(
           create: (context) => SettingsProvider(context.read<SharedPreferences>()),
         ),
@@ -23,7 +25,7 @@ void main() async {
           create: (_) => GpsDataManager(),
         ),
         ChangeNotifierProxyProvider2<GpsDataManager, SettingsProvider, OverlayProvider>(
-          create: (_) => OverlayProvider(),
+          create: (context) => OverlayProvider(context.read<OverlayService>()),
           update: (_, gpsManager, settings, overlay) => overlay!
             ..updateDependencies(
               gpsManager: gpsManager,