implemented comprehensive accessibility (A11y) support across the app

lib/view/pages/timetable/details/lesson_sheet.dart

@@ -103,6 +103,7 @@ class LessonSheet {
   static Widget _roomTile(BuildContext context, McTimetableEntry lesson) {
     final trailing = IconButton(
       icon: const Icon(Icons.house_outlined),
+      tooltip: 'Raumplan öffnen',
       onPressed: () => AppRoutes.openRoomplan(context),
     );