feat(04-01): NotificationSettingsNotifier with SharedPreferences persistence and full test suite

- Fix NotificationService API calls to use flutter_local_notifications v21 named parameters
- Expose nextInstanceOf as @visibleForTesting public method for unit testing
- Create NotificationSettingsNotifier with @Riverpod(keepAlive: true)
- NotificationSettings data class with enabled bool + TimeOfDay fields
- Persist notifications_enabled, notifications_hour, notifications_minute to SharedPreferences
- Sync default state in build(), async _load() overrides on hydration
- Update tests to use correct Riverpod 3 provider name (notificationSettingsProvider)
- Add makeContainer() helper to await initial _load() before asserting mutations
- All 84 tests pass (72 existing + 12 new notification tests)

test/core/notifications/notification_settings_notifier_test.dart

@@ -5,6 +5,16 @@ import 'package:shared_preferences/shared_preferences.dart';
 
 import 'package:household_keeper/core/notifications/notification_settings_notifier.dart';
 
+/// Helper: create a container and wait for the initial async _load() to finish.
+Future<ProviderContainer> makeContainer() async {
+  final container = ProviderContainer();
+  // Trigger build
+  container.read(notificationSettingsProvider);
+  // Allow the async _load() to complete
+  await Future<void>.delayed(Duration.zero);
+  return container;
+}
+
 void main() {
   setUp(() {
     SharedPreferences.setMockInitialValues({});
@@ -15,32 +25,32 @@ void main() {
       final container = ProviderContainer();
       addTearDown(container.dispose);
 
-      final state = container.read(notificationSettingsNotifierProvider);
+      final state = container.read(notificationSettingsProvider);
 
       expect(state.enabled, isFalse);
       expect(state.time, const TimeOfDay(hour: 7, minute: 0));
     });
 
     test('setEnabled(true) updates state.enabled to true', () async {
-      final container = ProviderContainer();
+      final container = await makeContainer();
       addTearDown(container.dispose);
 
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(true);
 
       expect(
-        container.read(notificationSettingsNotifierProvider).enabled,
+        container.read(notificationSettingsProvider).enabled,
         isTrue,
       );
     });
 
     test('setEnabled(true) persists to SharedPreferences', () async {
-      final container = ProviderContainer();
+      final container = await makeContainer();
       addTearDown(container.dispose);
 
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(true);
 
       final prefs = await SharedPreferences.getInstance();
@@ -48,32 +58,32 @@ void main() {
     });
 
     test('setEnabled(false) updates state.enabled to false', () async {
-      final container = ProviderContainer();
+      final container = await makeContainer();
       addTearDown(container.dispose);
 
       // First enable, then disable
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(true);
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(false);
 
       expect(
-        container.read(notificationSettingsNotifierProvider).enabled,
+        container.read(notificationSettingsProvider).enabled,
         isFalse,
       );
     });
 
     test('setEnabled(false) persists to SharedPreferences', () async {
-      final container = ProviderContainer();
+      final container = await makeContainer();
       addTearDown(container.dispose);
 
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(true);
       await container
-          .read(notificationSettingsNotifierProvider.notifier)
+          .read(notificationSettingsProvider.notifier)
           .setEnabled(false);
 
       final prefs = await SharedPreferences.getInstance();
@@ -83,15 +93,15 @@ void main() {
     test(
       'setTime(09:30) updates state.time and persists hour+minute',
       () async {
-        final container = ProviderContainer();
+        final container = await makeContainer();
         addTearDown(container.dispose);
 
         await container
-            .read(notificationSettingsNotifierProvider.notifier)
+            .read(notificationSettingsProvider.notifier)
             .setTime(const TimeOfDay(hour: 9, minute: 30));
 
         expect(
-          container.read(notificationSettingsNotifierProvider).time,
+          container.read(notificationSettingsProvider).time,
           const TimeOfDay(hour: 9, minute: 30),
         );
 
@@ -110,16 +120,10 @@ void main() {
           'notifications_minute': 15,
         });
 
-        final container = ProviderContainer();
+        final container = await makeContainer();
         addTearDown(container.dispose);
 
-        // Trigger build
-        container.read(notificationSettingsNotifierProvider);
-
-        // Wait for async _load() to complete
-        await Future<void>.delayed(Duration.zero);
-
-        final state = container.read(notificationSettingsNotifierProvider);
+        final state = container.read(notificationSettingsProvider);
         expect(state.enabled, isTrue);
         expect(state.time, const TimeOfDay(hour: 8, minute: 15));
       },