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/theme/preview_data.ex

@@ -366,20 +366,7 @@ defmodule SimpleshopTheme.Theme.PreviewData do
   end
 
   defp get_real_categories do
-    Products.list_products(visible: true, status: "active")
-    |> Enum.map(& &1.category)
-    |> Enum.reject(&is_nil/1)
-    |> Enum.frequencies()
-    |> Enum.map(fn {name, count} ->
-      %{
-        id: Slug.slugify(name),
-        name: name,
-        slug: Slug.slugify(name),
-        product_count: count,
-        image_url: nil
-      }
-    end)
-    |> Enum.sort_by(& &1.name)
+    Products.list_categories()
   end
 
   defp mock_products do