feat: add React metabox island for coupon editor

- 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

build/admin.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash', 'wp-primitives'), 'version' => '83e1a34ae6a628833355');
+<?php return array('dependencies' => array('react', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash', 'wp-primitives'), 'version' => 'f1f91f1cc72d44a0469e');