extract shared theme CSS into standalone bundle

Move the 3 theme layer imports (primitives, layer2-attributes, semantic)
out of admin.css and shop.css into a new theme.css bundle loaded by all
root layouts. Eliminates 28 KB of duplication on admin pages where both
admin.css and shop.css were each embedding the same theme CSS.

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

assets/css/admin.css

@@ -3,11 +3,6 @@
 /* Reset */
 @import "./admin/reset.css";
 
-/* Theme CSS layers (used by theme editor preview) */
-@import "./theme-primitives.css";
-@import "./theme-layer2-attributes.css";
-@import "./theme-semantic.css";
-
 /* Admin components, layout, icons, transitions */
 @import "./admin/components.css";
 @import "./admin/layout.css";