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/src/app/projects/page.tsx

@@ -0,0 +1,7 @@
+"use client";
+
+import { ClientRoot } from "@/app/root";
+
+export default function ProjectsPage() {
+  return <ClientRoot />;
+}