[rewrite] Added shadcn ui to frontend and slightly tweaked backend

frontend/src/routes/(errors)/404.lazy.tsx

@@ -0,0 +1,6 @@
+import { createLazyFileRoute } from '@tanstack/react-router'
+import NotFoundError from '@/features/errors/not-found-error'
+
+export const Route = createLazyFileRoute('/(errors)/404')({
+  component: NotFoundError,
+})