Theme browser: Inconsistent behavior of navigation when on the first theme

[afercia]

Discovered while reviewing ​https://github.com/WordPress/wordpress-develop/pull/11560 / https://core.trac.wordpress.org/ticket/63760

Occurs only in the WP Admin > Appearance, theme browser. Does not occur in the Customize theme browser.

• Go to wp-admin/themes.php.
• Click on any theme 'Theme Details' (note that it appears on hover).
• The theme browser modal dialog opens.
• Press the Right arrow key to navigate to the next theme.
• Once on the last theme, observe that the Right arrow key doesn't do anything.
• Press the Left arrow key to navigate to the previous theme.
• Once on the first theme, observe that the Left arrow key re-starts navigation from the last theme although the Previous button with the left chevron icon looks disabled to indicate the theme browser is at the beginning of the themes collection.

Compare with the behavior in the Customizer theme browser:

• Activate a Theme to access the Customizer, for example: Twenty Twenty-One.
• Access the Customizer.
• In the left panel, click 'Change' to open the customize theme browser.
• Click on any theme 'Theme Details' (note that it appears on hover).
• Test the navigation with arrow keys as described above.
• Observe the left arrow key doesn't do anything when on the first theme, as expected.

Trac ticket: https://core.trac.wordpress.org/ticket/65715

## Use of AI Tools

None

[afercia]

In ​https://github.com/WordPress/wordpress-develop/pull/12696 I'm adding a simple check to bail early previous navigation when on the first theme.
It's a very small, self-contained, change. Moving to 7.1 consideration.