Drop hardcoded white backgrounds

Product and collection card images, gallery slides, the zoom overlay
image, and the products section were pinned to bg-white while every
other section uses bg-background. They are transparent now and inherit
the page.

Skeleton fills and image-placeholder tiles keep their zinc backgrounds —
those are load-state and empty-state affordances, not theming.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

components/shopify/collection-card.tsx

@@ -25,7 +25,7 @@ const CollectionCard: React.FC<CollectionCardProps> = ({ collection }) => {
       className="group block h-full"
     >
       {/* Collection Image */}
-      <div className="relative aspect-square overflow-hidden bg-white">
+      <div className="relative aspect-square overflow-hidden">
         {collection.image ? (
           <img
             src={collection.image.url}