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/Admin/AppearanceController.php

@@ -381,6 +381,7 @@ class AppearanceController
                         'image_position' => sanitize_text_field($data['layout']['image_position'] ?? 'left'),
                         'gallery_style' => sanitize_text_field($data['layout']['gallery_style'] ?? 'thumbnails'),
                         'sticky_add_to_cart' => (bool) ($data['layout']['sticky_add_to_cart'] ?? false),
+                        'layout_style' => sanitize_text_field($data['layout']['layout_style'] ?? 'flat'),
                     ],
                     'elements' => [
                         'breadcrumbs' => (bool) ($data['elements']['breadcrumbs'] ?? true),
@@ -601,7 +602,11 @@ class AppearanceController
                         'show_icon' => true,
                     ],
                 ],
-                'product' => [],
+                'product' => [
+                    'layout' => [
+                        'layout_style' => 'flat',
+                    ],
+                ],
                 'cart' => [],
                 'checkout' => [],
                 'thankyou' => [],