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/package.json

@@ -24,6 +24,7 @@
         "@radix-ui/react-slot": "^1.2.3",
         "@radix-ui/react-switch": "^1.2.6",
         "@radix-ui/react-tabs": "^1.1.13",
+        "@tailwindcss/typography": "^0.5.19",
         "@tanstack/react-query": "^5.90.5",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",