feat(ui): path-based app-router routes with 404 and project auth preview

Resolve routes from pathname (and legacy hash), add a notFound route with
a dedicated 404 page, and show a blurred workspace preview for
unauthenticated project visits. Wire up Next.js app-router entry files and
listen for popstate so history navigation updates the route.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.