replace Tailwind utilities in product + cart components with CSS (Phase 5b)

Remove ~140 Tailwind utility classes from product.ex and cart.ex, replacing
with semantic CSS classes in components.css. Delete helper functions that
generated Tailwind class strings (card_classes, image_container_classes,
content_padding_class, title_classes, hero_cta_classes, grid_classes).
Use data-* attributes for variant styling, grid columns, and sticky
positioning. Update theme-layer2 selectors for renamed classes.

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

lib/simpleshop_theme_web/components/page_templates/error.html.heex

@@ -18,7 +18,7 @@
         mode={@mode}
       />
 
-      <.product_grid columns={:fixed_4} gap="gap-4" class="mt-12 max-w-xl mx-auto">
+      <.product_grid columns={:fixed_4}>
         <%= for product <- Enum.take(assigns[:products] || [], 4) do %>
           <.product_card
             product={product}