feat: #3591771 Use UIkit Admin as the administration theme

Makes UIkit Admin the administration theme of Web Admin, instead of Default Admin.

Work item: #3591771 (closed)

• Require drupal/uikit_admin: ~4.0; the default recipe installs it with its blocks and sets it as the admin theme. Claro stays installed for the contrib screens built on its styles.
• A theme negotiator shows the log in, password reset and registration screens in UIkit Admin (its sign-in page) when it is the admin theme.
• webship-js: the admin theme feature checks uikit_admin; a new feature checks the sign-in screens.

AI-Generated: Yes

Checkpoints

• File an issue
• Addition for a new supported feature
• Automated functional testing coverage
• Documentation
• CI pipeline green
• Release