complete admin CSS refactor: delete utilities.css, add layout primitives

- Delete utilities.css (701 lines / 24 KB of Tailwind utility clones)
- Add layout.css with admin-stack, admin-row, admin-cluster, admin-grid
  primitives and gap variants (sm, md, lg, xl)
- Add transitions.css import and layout.css import to admin.css entry point
- Replace all Tailwind utility classes across 26 admin templates with
  semantic admin-*/theme-*/page-specific CSS classes
- Replace all non-dynamic inline styles with semantic classes
- Add ~100 new semantic classes to components.css (analytics, dashboard,
  order detail, settings, theme editor, generic utilities)
- Fix stray text-error → admin-text-error in media.ex
- Add missing .truncate definition to admin CSS
- Only remaining inline styles are dynamic data values (progress bars,
  chart dimensions) and one JS.toggle target

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/admin.css

@@ -8,10 +8,11 @@
 @import "./theme-layer2-attributes.css";
 @import "./theme-semantic.css";
 
-/* Admin components, icons, and utilities */
+/* Admin components, layout, icons, transitions */
 @import "./admin/components.css";
+@import "./admin/layout.css";
 @import "./admin/icons.css";
-@import "./admin/utilities.css";
+@import "./admin/transitions.css";
 
 /* LiveView loading state variants */
 .phx-click-loading, .phx-submit-loading, .phx-change-loading {