fix category images, gallery reset, scroll hint and orphan image cleanup

- category_nav pulls first product image per category from DB
- ProductImageScroll JS hook resets to index 0 on updated()
- collection filter bar gets CSS fade gradient scroll hint on mobile
- sync_product_images and delete_product_images now clean up orphaned
  Media.Image records to prevent DB bloat from repeated syncs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/theme-semantic.css

@@ -206,6 +206,22 @@
   }
 }
 
+/* Horizontal scroll fade hint for collection category pills (mobile only) */
+.collection-filter-scroll {
+  mask-image: linear-gradient(to right, black calc(100% - 2rem), transparent);
+  -webkit-mask-image: linear-gradient(to right, black calc(100% - 2rem), transparent);
+  scrollbar-width: none;
+
+  &::-webkit-scrollbar {
+    display: none;
+  }
+
+  @media (min-width: 640px) {
+    mask-image: none;
+    -webkit-mask-image: none;
+  }
+}
+
 /* Skip link for keyboard navigation */
 .skip-link {
   position: absolute;