fix: Update npm to latest version and fix Tailwind v4 PostCSS configuration

- Install @tailwindcss/postcss for Tailwind v4 compatibility
- Update postcss.config.js to use new Tailwind PostCSS plugin
- Update npm to latest version in backend Dockerfile
- Fixes build failures with Next.js 16.1.6 and Tailwind CSS v4.2.0

frontend/postcss.config.js

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