fix: Add @tailwindcss/postcss dependency

- Required for Tailwind CSS v4 with PostCSS
- Fixes PostCSS plugin loading error in Docker build

postcss.config.cjs

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