Enable autofocus for 'Stundenzeit auswählen' dialog (#1267)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit


- **New Features**
- Enabled autofocus on the due date text field in the homework dialog to
enhance user interaction.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/lib/homework/homework_dialog/homework_dialog.dart

@@ -736,6 +736,7 @@ class _DueDateChipsState extends State<_DueDateChips> {
                 width: 50,
                 child: TextField(
                   key: HwDialogKeys.customLessonChipDialogTextField,
+                  autofocus: true,
                   maxLength: 2,
                   textAlign: TextAlign.end,
                   style: const TextStyle(