Add missing trailing slash on landingPage wp-admin path

[ironprogrammer]

What is this PR doing?

Adds a trailing slash to the blueprint's landingPage value.

What problem is it solving?

When using the WordPress PR previewer, after WP admin loads, all menu sidebar links redirect to the homepage.

The blueprint setting for landingPage is configured as /wp-admin. The missing trailing slash results in the menu sidebar links being incorrect, in turn causing link clicks to redirect to the homepage.

Navigating with the top admin bar links, or manually adding a slash to the Playground page path, can work around the issue.

How is the problem addressed?

The trailing slash allows the generated sidebar links to include/prepend /wp-admin/ to their links.

Compare this with the Gutenberg PR viewer, which includes the trailing slash.

Testing Instructions

1. Open https://playground.wordpress.net/wordpress.html.
2. Paste in a PR ID or URL and click "Go".
3. ✅ In the Playground that launches, the path in the Playground address bar should include a trailing slash: /wp-admin/.
4. ✅ Clicking admin sidebar menu items should link to the intended admin pages.

[brandonpayton]

I am able to reproduce this behavior. The general issue should be fixed with a redirect for all cases by #1417. While that PR is still outstanding, fixing this specific case seems like a fine idea 😄

[brandonpayton]

Thanks for the fix, @ironprogrammer! playground.wordpress.net has been updated with your change.