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>
