Upgrade Nuxt version and update configuration

- Updated Nuxt from version 3.21.1 to 4.3.1 in package.json for improved features and performance.
- Removed outdated TypeScript error suppression in nuxt.config.ts for cleaner code.
- Updated README.md to reflect the transition to Nuxt 4 and added instructions for post-upgrade steps.
- Modified error handling in middleware to use consistent error property names.

frontend-nuxt/package.json

@@ -20,7 +20,7 @@
   "dependencies": {
     "leaflet": "^1.9.4",
     "md5": "^2.3.0",
-    "nuxt": "^3.21.1",
+    "nuxt": "^4.3.1",
     "vue": "^3.5.13",
     "vue-router": "^4.5.0"
   },