simplified code across the app and consolidated duplicated helpers

lib/view/pages/grade_averages/grade_averages_view.dart

@@ -15,14 +15,14 @@ class GradeAveragesView extends StatelessWidget {
     create: (context) => GradeAveragesBloc(),
     child: BlocBuilder<GradeAveragesBloc, GradeAveragesState>(
       builder: (context, state) {
-        var bloc = context.watch<GradeAveragesBloc>();
+        final bloc = context.read<GradeAveragesBloc>();
 
         return Scaffold(
           appBar: AppBar(
             title: const Text('Notendurchschnittsrechner'),
             actions: [
               Visibility(
-                visible: bloc.state.grades.isNotEmpty,
+                visible: state.grades.isNotEmpty,
                 child: IconButton(
                   tooltip: 'Alle zurücksetzen',
                   onPressed: () => ConfirmDialog(
@@ -59,7 +59,7 @@ class GradeAveragesView extends StatelessWidget {
                     )
                     .toList(),
                 onSelected: (isMiddleSchool) {
-                  if (bloc.state.grades.isNotEmpty) {
+                  if (state.grades.isNotEmpty) {
                     ConfirmDialog(
                       title: 'Notensystem wechseln',
                       content: