Fix Tailwind CSS v4 PostCSS plugin configuration

- Add @tailwindcss/postcss dependency
- Update postcss.config.js to use @tailwindcss/postcss plugin instead of tailwindcss

Co-authored-by: Marco Acierno <[email protected]>

Author: claude[bot]

frontend/package.json

@@ -66,6 +66,7 @@
   "devDependencies": {
     "@biomejs/biome": "1.9.4",
     "@python-italia/eslint-config": "^1.0.13",
+    "@tailwindcss/postcss": "^4.1.12",
     "@tailwindcss/typography": "^0.5.9",
     "@testing-library/dom": "^8.1.0",
     "@testing-library/jest-dom": "^5.14.1",

frontend/postcss.config.js

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