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)

lib/core/notifications/notification_settings_notifier.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart' show TimeOfDay;
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+part 'notification_settings_notifier.g.dart';
+
+class NotificationSettings {
+  final bool enabled;
+  final TimeOfDay time;
+
+  const NotificationSettings({required this.enabled, required this.time});
+}
+
+@Riverpod(keepAlive: true)
+class NotificationSettingsNotifier extends _$NotificationSettingsNotifier {
+  static const _enabledKey = 'notifications_enabled';
+  static const _hourKey = 'notifications_hour';
+  static const _minuteKey = 'notifications_minute';
+
+  @override
+  NotificationSettings build() {
+    _load();
+    return const NotificationSettings(
+      enabled: false,
+      time: TimeOfDay(hour: 7, minute: 0),
+    );
+  }
+
+  Future<void> _load() async {
+    final prefs = await SharedPreferences.getInstance();
+    final enabled = prefs.getBool(_enabledKey) ?? false;
+    final hour = prefs.getInt(_hourKey) ?? 7;
+    final minute = prefs.getInt(_minuteKey) ?? 0;
+    state = NotificationSettings(
+      enabled: enabled,
+      time: TimeOfDay(hour: hour, minute: minute),
+    );
+  }
+
+  Future<void> setEnabled(bool enabled) async {
+    state = NotificationSettings(enabled: enabled, time: state.time);
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setBool(_enabledKey, enabled);
+  }
+
+  Future<void> setTime(TimeOfDay time) async {
+    state = NotificationSettings(enabled: state.enabled, time: time);
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setInt(_hourKey, time.hour);
+    await prefs.setInt(_minuteKey, time.minute);
+  }
+}