feat: Upgrade to Tailwind CSS v4.1.18

- Migrate from tailwindcss v3.3 to v4.1.18
- Replace @tailwind directives with @import 'tailwindcss'
- Move custom colors to @theme block in globals.css
- Convert custom utilities to @utility syntax
- Update PostCSS config to use @tailwindcss/postcss
- Remove autoprefixer (now built-in)

postcss.config.js

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