feat(iap): remove_ads purchase/restore service + adsRemoved notifier

lib/state/ads_notifier.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'providers.dart';
+
+/// Whether the player owns the remove-ads entitlement. Seeded from the save
+/// repository; [grant] flips it on (after a successful purchase/restore) and
+/// persists.
+class AdsRemovedNotifier extends Notifier<bool> {
+  @override
+  bool build() => ref.read(saveRepositoryProvider).adsRemoved;
+
+  Future<void> grant() async {
+    if (state) return;
+    await ref.read(saveRepositoryProvider).setAdsRemoved(true);
+    state = true;
+  }
+}

lib/state/providers.dart

@@ -6,6 +6,7 @@ import '../data/streak.dart';
 import '../game/models/season.dart';
 import '../services/analytics_service.dart';
 import '../services/audio_service.dart';
+import 'ads_notifier.dart';
 import 'endless_best_notifier.dart';
 import 'game_session_notifier.dart';
 import 'progress_notifier.dart';
@@ -71,6 +72,9 @@ final endlessBestProvider = NotifierProvider<EndlessBestNotifier, int>(
   EndlessBestNotifier.new,
 );
 
+final adsRemovedProvider =
+    NotifierProvider<AdsRemovedNotifier, bool>(AdsRemovedNotifier.new);
+
 final analyticsProvider = Provider<AnalyticsService>(
   (ref) => AnalyticsService(DebugAnalyticsBackend()),
 );