feat: add shadcn/ui + Tailwind CSS v4 + Huge Icons foundation

- Install tailwindcss, @tailwindcss/postcss, clsx, tailwind-merge, class-variance-authority
- Install @hugeicons/react for icons
- Install Radix UI primitives (switch, tabs, label, separator, select, dialog, checkbox, dropdown-menu, popover)
- Install sonner for toast notifications
- Create postcss.config.js with Tailwind v4 PostCSS plugin
- Create jsconfig.json with @ path alias for src/admin
- Create components.json for shadcn configuration
- Update webpack.config.js with @ resolve alias
- Create globals.css with Tailwind v4 CSS-first config + shadcn CSS variables
- Create cn() utility in lib/utils.js
- Create 17 shadcn UI components (button, input, label, checkbox, switch, tabs, alert, separator, badge, textarea, dialog, sonner, table, skeleton, select, dropdown-menu, popover)
- Create async confirm() utility replacing SweetAlert2
- Create toast utility wrapping sonner

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' => '1a2b3d6682654786741a');
+<?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' => '9a1a0e2d03b8d775e648');