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/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";