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/lib/sectionStyles.ts

@@ -12,7 +12,7 @@ export interface SectionStyleResult {
  */
 export function getSectionBackground(styles?: Record<string, any>): SectionStyleResult {
     if (!styles) {
-        return { style: {}, hasOverlay: false, overlayStyle: undefined };
+        return { style: {}, hasOverlay: false, overlayOpacity: 0 };
     }
 
     const bgType = styles.backgroundType || 'solid';
@@ -56,3 +56,30 @@ export function getSectionBackground(styles?: Record<string, any>): SectionStyle
 
     return { style, hasOverlay, overlayOpacity, backgroundImage };
 }
+
+/**
+ * Returns inner container class names for the three content width modes:
+ * - full: edge-to-edge, no max-width
+ * - contained: centered max-w-6xl (matches Product page / SPA default)
+ * - boxed: centered max-w-5xl, wrapped in a white rounded-2xl card (matches product accordion cards)
+ *
+ * For 'boxed', apply this to the inner container div; no extra wrapper needed.
+ */
+export function getContentWidthClasses(contentWidth?: string): string {
+    switch (contentWidth) {
+        case 'full':
+            return 'w-full px-4 md:px-8';
+        case 'boxed':
+            return 'container mx-auto px-4 max-w-5xl';
+        case 'contained':
+        default:
+            return 'container mx-auto px-4';
+    }
+}
+
+/**
+ * Returns whether the section uses the boxed (card) layout.
+ */
+export function isBoxedLayout(contentWidth?: string): boolean {
+    return contentWidth === 'boxed';
+}