implemented comprehensive accessibility (A11y) support across the app

lib/view/pages/marianum_message/marianum_message_list_view.dart

@@ -22,6 +22,7 @@ class MarianumMessageListView extends StatelessWidget {
         title: const Text('Marianum Message'),
         actions: [
           IconButton(
+            tooltip: 'Suchen',
             icon: const Icon(Icons.search),
             onPressed: () {
               final list = bloc.state.data?.messageList;

lib/view/pages/marianum_message/search_marianum_messages.dart

@@ -23,11 +23,16 @@ class SearchMarianumMessages extends SearchDelegate<MarianumMessage?> {
   @override
   List<Widget>? buildActions(BuildContext context) => [
     if (query.isNotEmpty)
-      IconButton(onPressed: () => query = '', icon: const Icon(Icons.clear)),
+      IconButton(
+        tooltip: 'Leeren',
+        onPressed: () => query = '',
+        icon: const Icon(Icons.clear),
+      ),
   ];
 
   @override
   Widget? buildLeading(BuildContext context) => IconButton(
+    tooltip: 'Zurück',
     icon: const Icon(Icons.arrow_back),
     onPressed: () => close(context, null),
   );