Add catch-all 404 route and NotFound component to frontend routing

apps/frontend/app/routes.ts

@@ -1,3 +1,7 @@
-import { type RouteConfig, index } from "@react-router/dev/routes";
+import { type RouteConfig, index, route } from '@react-router/dev/routes';
 
-export default [index("routes/home.tsx")] satisfies RouteConfig;
+export default [
+  index('routes/home.tsx'),
+  // catch-all 404 route
+  route('*', 'routes/404.tsx'),
+] satisfies RouteConfig;

apps/frontend/app/routes/404.tsx

@@ -0,0 +1,3 @@
+export default function NotFound() {
+  return <h1>404 - Not Found</h1>;
+}