feat: upgrade Tailwind CSS to v4.1.11

- Upgrade Tailwind CSS from 3.x to 4.x for latest features and performance
- Install @tailwindcss/postcss plugin for Tailwind CSS 4.0 compatibility
- Update CSS configuration to use new @import and @config syntax
- Update PostCSS configuration to use new plugin format
- Build working correctly with new Tailwind CSS engine

postcss.config.js

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