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/product-detail/product-detail-gallery.tsx

@@ -132,7 +132,7 @@ const ProductDetailGallery: React.FC<ProductDetailGalleryProps> = ({
             key={index}
             onClick={() => setZoomedIndex(index)}
             aria-label={`Zoom ${image.altText || 'product image'}`}
-            className={`relative aspect-square w-full shrink-0 snap-center overflow-hidden bg-white pointer-events-none sm:pointer-events-auto sm:shrink sm:cursor-zoom-in ${
+            className={`relative aspect-square w-full shrink-0 snap-center overflow-hidden pointer-events-none sm:pointer-events-auto sm:shrink sm:cursor-zoom-in ${
               isSingle ? 'sm:col-span-2' : ''
             }`}
           >
@@ -177,7 +177,7 @@ const ProductDetailGallery: React.FC<ProductDetailGalleryProps> = ({
             src={zoomedImage.url}
             alt={zoomedImage.altText || 'Product image'}
             onClick={(event) => event.stopPropagation()}
-            className="max-h-full max-w-full object-contain bg-white"
+            className="max-h-full max-w-full object-contain"
           />
 
           <Button