implemented comprehensive accessibility (A11y) support across the app

lib/view/pages/timetable/timetable.dart

@@ -110,6 +110,7 @@ class _TimetableState extends State<Timetable> {
         actions: [
           IconButton(
             icon: const Icon(Icons.home_outlined),
+            tooltip: 'Zur aktuellen Woche',
             onPressed: atToday ? null : _jumpToToday,
           ),
           PopupMenuButton<_CalendarAction>(
@@ -179,6 +180,7 @@ class _TimetableState extends State<Timetable> {
         actions: [
           IconButton(
             icon: const Icon(Icons.home_outlined),
+            tooltip: 'Zur aktuellen Woche',
             onPressed: atToday ? null : _jumpToToday,
           ),
           if (canViewForeign)