feat: Add UIkit Admin layouts for the view modes

Problem/Motivation

UIkit Admin 4.0.x is a standalone administration theme built with UIkit and single directory components (uikit_admin, #3626238). When content managed by VMI is shown in the back office (dashboards, previews) with UIkit Admin active, it falls back to the plain field layout because VMI has no template for that theme.

Steps to reproduce

1. Install a site with UIkit Admin as the administration theme.
2. Open a content preview or dashboard view that VMI manages.
3. The view mode renders as plain fields instead of a themed layout.

Proposed resolution

Ship src/assets/config_templates/uikit_admin with a template for every view mode. UIkit Admin has a card component but no overlay, cover or article component yet, so every view mode uses the card. Cover the templates in ThemeTemplatesTest, document the theme support in the README, and suggest drupal/uikit_admin in composer.json.

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

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A

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