Default keyboard: Focus indicator is not visible on "Update project thumbnail" link button in Share Project dialog

Describe the bug
When navigating via keyboard, the focus indicator is not visible when focus moves to the "Update project thumbnail" link button in the Share Project dialog.

Steps to reproduce the behavior:

1. Navigate to https://makecode.microbit.org/beta.
2. Open an existing project or create a new one.
3. Use Tab to move focus to the Share Project button.
4. Press Enter to open the Share Project dialog.
5. Continue pressing Tab until focus reaches the "Update project thumbnail" link button.
6. Observe the focus state.

Actual behavior
No visible focus indicator (focus border) is displayed when the "Update project thumbnail" link button receives focus.

Expected behavior
A clearly visible focus indicator should be displayed when the "Update project thumbnail" link button is focused, providing proper keyboard navigation feedback and ensuring accessibility compliance (e.g., WCAG 2.1).

Screen Recording:
https://github.com/user-attachments/assets/087dfc6a-8101-4d86-a803-5232cd419738

Additional context
1.OS: Windows 11 25H2
2.Browser: Chrome, Edge, and Firefox
3.makecode.microbit.org version: 8.1.17
4.Microsoft MakeCode version: 12.3.12

Reproduce the issue at makecode.microbit.org/beta by opening the Share Project dialog and tabbing to the “Update project thumbnail” link button. Inspect the dialog’s focus styling for that control and verify completion by confirming a clearly visible focus indicator in Chrome, Edge, and Firefox.