claude refactorings, flutter best practices, platform dependent changes, general cleanup

lib/view/pages/timetable/widgets/appointment_tile.dart

@@ -56,7 +56,7 @@ class AppointmentTile extends StatelessWidget {
           ),
           if (crossedOut)
             Positioned.fill(
-              child: Container(
+              child: DecoratedBox(
                 decoration: BoxDecoration(
                   border: Border.all(width: 2, color: Colors.red.withAlpha(200)),
                   borderRadius: const BorderRadius.all(Radius.circular(7)),