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/layout.css

@@ -24,6 +24,15 @@
     align-items: center;
   }
 
+  /* Stack gap variants */
+  .admin-stack-sm { --admin-stack-gap: 0.5rem; }
+  .admin-stack-md { --admin-stack-gap: 0.75rem; }
+  .admin-stack-lg { --admin-stack-gap: 1.5rem; }
+  .admin-stack-xl { --admin-stack-gap: 2rem; }
+
+  /* Row gap variants */
+  .admin-row-lg { --admin-row-gap: 0.75rem; }
+
   /* Intrinsic responsive grid — cards, media thumbnails */
   .admin-grid {
     display: grid;