fix: #3631526 Follow the active admin theme: Claro, Gin and Default Admin

Part of #3631526.

The AI tabs now stay inside the active admin theme and use core classes: Claro (core), Gin (contrib) and Default Admin (core, experimental).

• Tokens fall back Default Admin --admin-*, then Gin --gin-*, then Claro, then a literal. Dark mode follows the theme via light-dark() and color-scheme, no theme class.
• Custom surface, accent and colour rules removed. The error label uses each theme's own error text token (fixes 3.72:1 in Default Admin dark).
• The "New" badge follows the themes' native new flag. "Generate again" is core button--small. JS clears errors through core's [data-drupal-messages].
• Unused ai-media__help and ai-media__shape classes and the xxxs cspell word removed.

axe: 0 violations in all three themes, light and dark, at 1440 and 1920. phpcs, stylelint, cspell clean; eslint 0 errors.

Remaining tasks

• Image preview and a successful SVG not tested: the Echo provider only does chat.
• In Gin and Default Admin the Options panel is up to 16px (Gin) or 290px (Default Admin) wider than the textarea, from the theme's form item margin and textarea cols. Left as is.

AI-Generated: Yes

Checkpoints:

• 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
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release