feat: rewrite CouponMetabox with proper shadcn/ui

- Remove all WPCFTO wrapper components (MetaboxLayout, TabNav, TabPanel, GroupTitle, etc.)
- Use shadcn Tabs with horizontal underline triggers
- Use grid-cols-[1fr_2fr] for clean form field layout
- Use shadcn Switch for toggle fields
- Use shadcn Select for discount type dropdown
- Replace inline Notice with toast notifications
- Add Skeleton loading state
- Add SearchableSelect with Badge chips for relation fields
- Move save button to tab header bar
- No custom CSS classes - all Tailwind utilities

build/admin.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'react-dom', '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' => '6bf4643f2ea15ecdf0b7');
+<?php return array('dependencies' => array('react', 'react-dom', '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' => 'f5d52a62f3d00d92014a');