Custom Frutiger Aero 404

app/not-found.tsx renders a standalone screen: the root layout's ice
scene is the backdrop, the glass card/rim/bubbles come from the shared
recipes (full Aero with that theme on, frosted glass in light/dark),
and the mascot is the scene's glossy ice orb at full strength with a
snowflake and a "404" chip. Two ways out - back to the library and
into the chat - both 44px targets, en + cs.

The regression suite pins the 404 status, both links and the Czech
copy; the pixel-contrast scan covers the screen in all three themes.
The service worker no longer caches non-OK navigations as the offline
shell (a typo'd URL used to write the error page into the shell
cache). Version 0.5.6, SW -> freezmo-v78.

app/not-found.tsx

@@ -0,0 +1,12 @@
+import type { Metadata } from "next";
+
+import { NotFoundScreen } from "@/components/NotFoundScreen";
+
+export const metadata: Metadata = {
+  title: "Page not found",
+};
+
+/** Custom 404 for unknown URLs (and notFound() calls). */
+export default function NotFound() {
+  return <NotFoundScreen />;
+}