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

postcss.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+    plugins: {
+        '@tailwindcss/postcss': {},
+        autoprefixer: {},
+    },
+};