fix: scope all Tailwind utilities under .formipay-design-system for WP admin isolation

- Add postcss-prefix-selector to prefix all CSS selectors with .formipay-design-system
- This gives Tailwind utilities higher specificity than WP admin styles
- Prevents WP input/select/button styles from overriding shadcn components
- All 129 selectors now scoped correctly

package.json

@@ -16,6 +16,7 @@
   },
   "devDependencies": {
     "@wordpress/scripts": "^27.0.0",
+    "postcss-prefix-selector": "^2.1.1",
     "tw-animate-css": "^1.4.0"
   },
   "dependencies": {