fix: #3626339 Meet WCAG 2.2 AAA and fix the page layout with UI Patterns, phones and messages

The second half of 4.0.1: WCAG 2.2 AAA, and the layout bugs a crawl of the Webship site templates found (every core and contrib admin screen, with UI Patterns installed).

Issue: https://www.drupal.org/project/uikit_admin/issues/3626339

Layout and behaviour

• Empty slots render nothing: no empty 25% sidebar column, no empty role=alert, description, prefix or suffix wrappers when UI Patterns is on.
• Tables scroll inside their own region instead of the page (1.4.10); webform, metatag and module lists included.
• Phones: a 44px rail toggle and search, no title/search overlap, the closed rail is inert and out of the tab order, focus returns to the toggle.
• Status messages get their own region (post_update moves the block); Drupal.Message renders UIkit alerts; the skip link is visible on focus; the CKEditor sticky toolbar stays under the header (Drupal.displace); the sticky Save row setting works.
• Theme settings have a config schema and install defaults (post_update casts stored values).
• The UI Skins color modes are uikit_admin_light / uikit_admin_dark (still data-theme="light|dark"), so they no longer replace the modes of UI Suite UIkit; post_update renames a stored mode.
• Dialogs fit the viewport and cover the rail; Coffee and the help block get named landmarks.

WCAG 2.2 AAA

• Tokens at 7:1 in light and dark (muted #454d57, accent #08508a, dark accent #8cbcff, control borders #7d8794, AAA alert colours, dark alerts); post_update moves a stored old default accent.
• One solid 2px focus ring everywhere; 44px targets (rail, top bar, buttons, breadcrumb, controls); description typography; links in text underlined; labels for title-only fields and empty table headers.
• UIkit rebuilt with its form icons inlined (white checks on the accent).
• axe with the AAA tags: 0 failing nodes on the admin screens of core, the CI-identical site and website_starter, light and dark, 1280 and 320px.

Tests

• 03-01-01-regressions-401.feature (10) and 03-02-01-wcag-aaa.feature (36); CI installs ui_patterns (require-dev) for the UI Patterns cases.
• 166 scenarios pass on the dev and CI-identical sites.

AI-Generated: Yes

Checkpoints

• File an issue
• Fix
• Automated functional testing coverage
• Accessibility
• Documentation
• CI pipeline green
• Release