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";

assets/css/shop.css

@@ -5,13 +5,6 @@
 
 @import "./shop/reset.css";
 
-/* Theme CSS — unlayered so it overrides component layers (intentional).
-   Primitives set CSS custom properties, layer2 sets theme-aware rules,
-   semantic sets base styles on .themed containers. */
-@import "./theme-primitives.css";
-@import "./theme-layer2-attributes.css";
-@import "./theme-semantic.css";
-
 @import "./shop/components.css";
 @import "./shop/layout.css";
 @import "./shop/utilities.css";

assets/css/theme.css

@@ -0,0 +1,6 @@
+/* Theme design tokens — shared by admin and shop CSS bundles.
+   Loaded separately so the browser caches it once for all page types. */
+
+@import "./theme-primitives.css";
+@import "./theme-layer2-attributes.css";
+@import "./theme-semantic.css";