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

postcss.config.js

@@ -1,6 +1,26 @@
+const prefixSelector = require('postcss-prefix-selector');
+
 module.exports = {
     plugins: {
         '@tailwindcss/postcss': {},
         autoprefixer: {},
+        'postcss-prefix-selector': {
+            prefix: '.formipay-design-system',
+            transform(prefix, selector) {
+                // Don't prefix root-level selectors like :root, @keyframes
+                if (selector.startsWith(':root') || selector.startsWith('@keyframes') || selector.startsWith('@font-face')) {
+                    return selector;
+                }
+                // Don't prefix html/body selectors
+                if (selector.startsWith('html') || selector.startsWith('body')) {
+                    return selector;
+                }
+                // Don't double-prefix
+                if (selector.startsWith(prefix)) {
+                    return selector;
+                }
+                return `${prefix} ${selector}`;
+            },
+        },
     },
 };