feat: Use UIkit Admin as the administration theme

Problem/Motivation

Sites installed from the Webship Starter recipe use Default Admin in the back office. UIkit Admin 4.0.x is now a standalone administration theme built with UIkit and single directory components (uikit_admin, #3626238).

Steps to reproduce

1. Install a site from the Webship Starter recipe.
2. Check Appearance > Administration theme.
3. Default Admin is set, not UIkit Admin.

Proposed resolution

Web Admin now requires and sets UIkit Admin as the administration theme, so the recipe picks it up automatically. Update the recipe comments and the install tests so a fresh install checks the admin theme is uikit_admin.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Merge request, Patch, or Commit
• ❌ Testing to ensure no regression
• ❌ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ❌ Documentation
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Release notes snippet
• ❌ Release

User interface changes

• Administration pages use UIkit Admin instead of Default Admin.

API changes

• N/A

Data model changes

• N/A

AI-Generated: Yes