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

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

lib/data/save_repository.dart

@@ -42,6 +42,9 @@ class SaveRepository {
       _adsRemoved =
           (json['flags'] as Map<String, dynamic>?)?['adsRemoved'] as bool? ??
               false;
+      _soundEnabled =
+          (json['flags'] as Map<String, dynamic>?)?['soundEnabled'] as bool? ??
+              true;
     }
   }
 
@@ -56,11 +59,13 @@ class SaveRepository {
   bool _tutorialDone = false;
   int _endlessBest = 0;
   bool _adsRemoved = false;
+  bool _soundEnabled = true;
 
   StreakState get streak => _streak;
   bool get tutorialDone => _tutorialDone;
   int get endlessBest => _endlessBest;
   bool get adsRemoved => _adsRemoved;
+  bool get soundEnabled => _soundEnabled;
 
   Future<void> markTutorialDone() {
     _tutorialDone = true;
@@ -72,6 +77,11 @@ class SaveRepository {
     return _flush();
   }
 
+  Future<void> setSoundEnabled(bool value) {
+    _soundEnabled = value;
+    return _flush();
+  }
+
   Future<void> recordEndlessScore(int score) {
     if (score > _endlessBest) _endlessBest = score;
     return _flush();
@@ -140,7 +150,11 @@ class SaveRepository {
             'best': _streak.best,
             'lastYmd': _streak.lastYmd,
           },
-          'flags': {'tutorialDone': _tutorialDone, 'adsRemoved': _adsRemoved},
+          'flags': {
+            'tutorialDone': _tutorialDone,
+            'adsRemoved': _adsRemoved,
+            'soundEnabled': _soundEnabled,
+          },
           'endless': {'best': _endlessBest},
         }),
       );