feat: Use UIkit Admin as the theme of Cucumber

Problem/Motivation

Cucumber uses Gin as both the front-end and the administration theme. 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 Cucumber Starter recipe.
2. Check Appearance > default theme and administration theme.
3. Both are Gin.

Proposed resolution

Require drupal/uikit_admin ~4.0 instead of drupal/gin, install it, and set it as both the default and the administration theme, since Cucumber is an administration application. Update the recipe tests to match.

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

• Both the default and administration theme change from Gin to UIkit Admin.

API changes

• N/A

Data model changes

• N/A

AI-Generated: Yes