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

includes/Api/OnboardingController.php

@@ -122,7 +122,7 @@ class OnboardingController extends WP_REST_Controller
         }
 
         // 4. Mark as Complete
-        update_option('woonoow_onboarding_completed', true);
+        update_option('woonoow_onboarding_completed', 'yes');
 
         return rest_ensure_response([
             'success' => true,