feat: Rebuild UIkit Admin as a standalone administration theme built with components

Problem/Motivation

UIkit Admin is a sub-theme of the UIkit theme and stopped at Drupal 9. It cannot follow the administration themes of core, and its markup lives in a long list of template overrides.

Steps to reproduce

N/A

Proposed resolution

A new 4.0.x branch for Drupal 11.4 and 12. The theme stands on its own, with no base theme. Every part of the back office is a single directory component: the sidebar, the top bar with a keyboard palette, the page shell, tabs, actions, operations, forms, tables and the status report. Content forms keep their meta in a sidebar. The colours meet the WCAG AA contrast minimum, with a dark mode and a compact density. UI Skins and UI Styles are supported when they are installed. A webship-js suite covers the screens and their accessibility.

Remaining tasks

• ☑ File an issue
• ☐ Addition/Change/Update/Fix
• ☐ 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
• ☐ Release

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A

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