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/products.tsx

@@ -172,7 +172,7 @@ const Products: React.FC<ProductsProps> = ({
   }
 
   return (
-    <div className="py-20 bg-white">
+    <div className="py-20">
       <div className="max-w-screen-2xl mx-auto px-8">
         <h2 className="text-center text-2xl md:text-3xl font-normal max-w-3xl mx-auto text-foreground">
           {title}