Menu Bar: Implement File -> Exit (Close Window)

The `File -> Exit` button in the top menu bar currently does not perform any action when pressed.

**Proposed Solution:**
Modify `lib/features/main_screen/querya_window_title_bar.dart` and bind the `onPressed` callback to `appWindow.close()` from the `bitsdojo_window` package:
```dart
MenuButton(
  onPressed: (_) => appWindow.close(),
  child: const Text('Exit'),
)
```