Add a minimal 404 page matching the app's style

Two boundaries share the same content: src/app/not-found.tsx catches
genuinely unmatched URLs (rendered bare in the root layout), and
(app)/not-found.tsx catches notFound() calls from within app routes
(already used by scripts/[scriptId] and runs/[runId]) so it renders
nested inside AppLayout, keeping nav and footer visible.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/not-found.tsx

@@ -0,0 +1,5 @@
+import { NotFoundContent } from "@/components/layout/not-found-content";
+
+export default function NotFound() {
+  return <NotFoundContent />;
+}