implemented comprehensive accessibility (A11y) support across the app

lib/widget/file_viewer.dart

@@ -249,6 +249,7 @@ class _FileViewerState extends State<FileViewer> {
               photoViewController.rotation += pi / 2;
             });
           },
+          tooltip: 'Drehen',
           icon: const Icon(Icons.rotate_right),
         ),
       ],