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;
+  }
+}