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
90169b508d