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>

lib/simpleshop_theme_web/live/shop/collection.ex

@@ -135,7 +135,10 @@ defmodule SimpleshopThemeWeb.Shop.Collection do
   defp collection_filter_bar(assigns) do
     ~H"""
     <div class="flex flex-wrap items-center justify-between gap-3 mb-6">
-      <nav aria-label="Collection filters" class="overflow-x-auto -mx-4 px-4 sm:mx-0 sm:px-0">
+      <nav
+        aria-label="Collection filters"
+        class="collection-filter-scroll overflow-x-auto py-1 -mx-4 px-4 sm:overflow-visible sm:mx-0 sm:px-0 sm:py-0"
+      >
         <ul class="flex gap-1.5 sm:flex-wrap sm:gap-2">
           <li class="shrink-0">
             <.link