Refactor code structure for improved readability and maintainability

Co-authored-by: Copilot <copilot@github.com>

flutter/lib/features/meal_plan/data/meal_plan_providers.dart

@@ -11,7 +11,15 @@ final mealPlanRepositoryProvider = Provider<MealPlanRepository>((ref) {
   return MealPlanRepository(ref.watch(apiClientProvider));
 });
 
-final mealPlanWeekOffsetProvider = StateProvider<int>((ref) => 0);
+class _IntNotifier extends Notifier<int> {
+  _IntNotifier(this._initial);
+  final int _initial;
+  @override
+  int build() => _initial;
+}
+
+final mealPlanWeekOffsetProvider =
+    NotifierProvider<_IntNotifier, int>(() => _IntNotifier(0));
 
 final mealPlanWeekProvider = Provider<MealPlanWeek>((ref) {
   final offset = ref.watch(mealPlanWeekOffsetProvider);