implemented comprehensive accessibility (A11y) support across the app

lib/view/pages/grade_averages/grade_averages_list_view.dart

@@ -35,6 +35,7 @@ class GradeAveragesListView extends StatelessWidget {
                   Text(getGradeDisplay(grade)),
                   const SizedBox(width: 30),
                   IconButton(
+                    tooltip: 'Note entfernen',
                     onPressed: () {
                       bloc.add(DecrementGrade(grade));
                     },
@@ -49,6 +50,7 @@ class GradeAveragesListView extends StatelessWidget {
                     ),
                   ),
                   IconButton(
+                    tooltip: 'Note hinzufügen',
                     onPressed: () {
                       bloc.add(IncrementGrade(grade));
                     },
@@ -64,6 +66,7 @@ class GradeAveragesListView extends StatelessWidget {
               maintainSize: true,
               visible: bloc.canDecrementOrDelete(grade),
               child: IconButton(
+                tooltip: 'Löschen',
                 icon: const Icon(Icons.delete),
                 onPressed: () {
                   bloc.add(ResetGrade(grade));

lib/view/pages/grade_averages/grade_averages_view.dart

@@ -24,6 +24,7 @@ class GradeAveragesView extends StatelessWidget {
               Visibility(
                 visible: bloc.state.grades.isNotEmpty,
                 child: IconButton(
+                  tooltip: 'Alle zurücksetzen',
                   onPressed: () => ConfirmDialog(
                     title: 'Zurücksetzen?',
                     content: 'Alle Einträge werden entfernt.',