refactor: streamline settings bindings

lib/mixins/settings_effect_mixin.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/widgets.dart';
+
+import '../services/settings_service.dart';
+
+/// Wires up settings → side-effect bindings without manual addListener/dispose.
+///
+/// In `initState`:
+///   bindEffect(SettingsService.rotationLocked, _applyRotation);
+///   bindEffect(SettingsService.audioSyncOffset, (v) => player.setAudioDelay(v));
+///
+/// The callback fires immediately with the current value (unless
+/// `fireImmediately: false`) so apply-on-init wiring stays in one place, and
+/// then on every subsequent write — no `didChangeAppLifecycleState` reload.
+mixin SettingsEffectMixin<T extends StatefulWidget> on State<T> {
+  final List<VoidCallback> _settingsEffectDisposers = [];
+
+  /// Subscribe to changes of [pref] and run [effect]. Auto-disposed in [dispose].
+  void bindEffect<V>(Pref<V> pref, void Function(V value) effect, {bool fireImmediately = true}) {
+    final notifier = SettingsService.instanceOrNull!.listenable(pref);
+    void listener() => effect(notifier.value);
+    notifier.addListener(listener);
+    _settingsEffectDisposers.add(() => notifier.removeListener(listener));
+    if (fireImmediately) effect(notifier.value);
+  }
+
+  /// Rebuild this widget when any of [prefs] changes. Use for state classes
+  /// that synthesize multiple settings into derived getters and need their
+  /// build to refresh on any change. Equivalent to wrapping the widget tree
+  /// in a [SettingsBuilder], but lets you keep raw `setState`-style state too.
+  void bindRebuild(List<Pref<Object?>> prefs) {
+    final svc = SettingsService.instanceOrNull!;
+    final merged = Listenable.merge(prefs.map(svc.listenableOf).toList(growable: false));
+    void listener() {
+      if (mounted) setState(() {});
+    }
+
+    merged.addListener(listener);
+    _settingsEffectDisposers.add(() => merged.removeListener(listener));
+  }
+
+  @override
+  void dispose() {
+    for (final d in _settingsEffectDisposers) {
+      d();
+    }
+    _settingsEffectDisposers.clear();
+    super.dispose();
+  }
+}