feat(settings): soundEnabled provider gates SFX and haptics

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

lib/state/sound_notifier.dart

@@ -0,0 +1,17 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'providers.dart';
+
+/// SFX + gameplay haptics on/off, seeded from the save repository.
+class SoundEnabledNotifier extends Notifier<bool> {
+  @override
+  bool build() => ref.read(saveRepositoryProvider).soundEnabled;
+
+  Future<void> toggle() => set(!state);
+
+  Future<void> set(bool value) async {
+    if (state == value) return;
+    await ref.read(saveRepositoryProvider).setSoundEnabled(value);
+    state = value;
+  }
+}