fix: Use @tailwindcss/postcss for Tailwind 4

- Install @tailwindcss/postcss package
- Update postcss.config.cjs to use new plugin
- Required for Tailwind CSS v4

postcss.config.cjs

@@ -1,6 +1,5 @@
 module.exports = {
   plugins: {
     tailwindcss: {},
-    autoprefixer: {},
   },
 };