feat(settings): persist soundEnabled flag (additive, default true)

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

test/data/save_repository_sound_test.dart

@@ -0,0 +1,24 @@
+import 'package:block_seasons/data/save_repository.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+void main() {
+  setUp(() => SharedPreferences.setMockInitialValues({}));
+
+  test('soundEnabled defaults true and persists across reopen', () async {
+    final repo = await SaveRepository.open();
+    expect(repo.soundEnabled, isTrue);
+    await repo.setSoundEnabled(false);
+    expect(repo.soundEnabled, isFalse);
+    final reopened = await SaveRepository.open();
+    expect(reopened.soundEnabled, isFalse);
+  });
+
+  test('legacy save without the sound flag reads as true', () async {
+    SharedPreferences.setMockInitialValues({
+      'save_v1': '{"saveVersion":1,"progress":{},"flags":{"tutorialDone":true}}',
+    });
+    final repo = await SaveRepository.open();
+    expect(repo.soundEnabled, isTrue);
+  });
+}