WPF gallery caption buttons are focusable

The caption buttons of the WPF gallery app can be selected with the tab key, and even show a focus visual. Caption buttons should not be focusable, or selectable with the tab key.

<img width="187" height="67" alt="Image" src="https://github.com/user-attachments/assets/e62b07d7-3376-4f8d-806a-73539319dd67" />