feat: product page layout toggle (flat/card), fix email shortcode rendering

- Add layout_style setting (flat default) to product appearance
  - AppearanceController: sanitize & persist layout_style, add to default settings
  - Admin SPA: Layout Style select in Appearance > Product
  - Customer SPA: useEffect targets <main> bg-white in flat mode (full-width),
    card mode uses per-section white floating cards on gray background
  - Accordion sections styled per mode: flat=border-t dividers, card=white cards

- Fix email shortcode gaps (EmailRenderer, EmailManager)
  - Add missing variables: return_url, contact_url, account_url (alias),
    payment_error_reason, order_items_list (alias for order_items_table)
  - Fix customer_note extra_data key mismatch (note → customer_note)
  - Pass low_stock_threshold via extra_data in low_stock email send

customer-spa/src/hooks/useAppearanceSettings.ts

@@ -121,6 +121,7 @@ export function useProductSettings() {
       image_position: 'left' as string,
       gallery_style: 'thumbnails' as string,
       sticky_add_to_cart: false,
+      layout_style: 'flat' as string,
     },
     elements: {
       breadcrumbs: true,