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

node_modules/.package-lock.json

@@ -17429,6 +17429,16 @@
         "postcss": "^8.4.31"
       }
     },
+    "node_modules/postcss-prefix-selector": {
+      "version": "2.1.1",
+      "resolved": "https://registry.npmjs.org/postcss-prefix-selector/-/postcss-prefix-selector-2.1.1.tgz",
+      "integrity": "sha512-ZBgf427Et6+XnrnJ9VXtJEKCjJwTvn2wn/qMg+wvvlRhIeFIAxdbrlZZ0CSsWYMJfcyPLBh8ogj5O1kb/Mcx3g==",
+      "dev": true,
+      "license": "MIT",
+      "peerDependencies": {
+        "postcss": "^8.0.0"
+      }
+    },
     "node_modules/postcss-reduce-initial": {
       "version": "6.1.0",
       "resolved": "https://registry.npmjs.org/postcss-reduce-initial/-/postcss-reduce-initial-6.1.0.tgz",