Add 404 and error boundary components wired into router

frontend/src/routes/$slug.tsx

@@ -1,6 +1,7 @@
 import { useSuspenseQuery } from '@tanstack/react-query'
 import { createFileRoute, notFound } from '@tanstack/react-router'
 import { Helmet } from 'react-helmet-async'
+import { NotFound } from '~/components/not-found'
 import { PageView } from '~/components/page-view'
 import { pageBySlugQuery } from '~/lib/queries'
 
@@ -10,6 +11,7 @@ export const Route = createFileRoute('/$slug')({
     if (!page) throw notFound()
   },
   component: CmsPage,
+  notFoundComponent: () => <NotFound />,
 })
 
 function CmsPage() {