fix: eliminate pull-to-refresh flicker by holding stale data during re-fetch

lib/providers/tasks_provider.dart

@@ -17,6 +17,11 @@ class TasksNotifier extends AsyncNotifier<List<Task>> {
     return ref.watch(tasksRepositoryProvider).getAll();
   }
 
+  Future<void> refresh() async {
+    final fresh = await ref.read(tasksRepositoryProvider).getAll();
+    state = AsyncData(fresh);
+  }
+
   Future<Task> create({
     required String title,
     String? description,