feat: #3626238 Rebuild UIkit Admin as a standalone administration theme built with components

Rebuilds UIkit Admin as a standalone administration theme (base theme: false), built on UIkit 3.25 with Single Directory Components.

It follows the logic of the UI Suite initiative and makes use of its smart parts, most of which have now moved into Drupal core: core SDC components, and CSS custom properties that UI Skins and UI Styles change on top of the core style and token APIs.

Issue: https://www.drupal.org/project/uikit_admin/issues/3626238

• Sidebar rail with Create / Build / Run sections, a sticky top bar and a Ctrl/Cmd+K command palette
• 59 SDC components; admin pages, forms, tables, status report and the node edit form rendered through them
• Appearance settings: accent and focus colors, light/dark/auto, comfortable/compact density
• UI Skins and UI Styles plugin files
• webship-js functional and axe accessibility tests, run in CI

AI-Generated: Yes

Checkpoints

• Tested on plain Drupal core 11.4
• Tested on website_starter
• webship-js suite passes locally (22 scenarios, 427 steps)
• CI pipeline green