- Create CouponMetabox React component with WPCFTO design system
- Add MetaboxLayout with vertical tabs (Rules, Restrictions)
- Implement Rules tab: active toggle, type radio, amount fields, multi-currency support
- Implement Restrictions tab: usage limit, date limit, autocomplete for forms/products/customers
- Add metabox registration in Coupon.php for formipay-coupon post type
- Update ReactAdmin to load assets on post.php edit screens
- Add autocomplete AJAX handler for relation fields
- Disable old WPCFTO metabox in favor of React island
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