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.

tests/contrast.mjs

@@ -44,6 +44,7 @@ const screens = [
   ["stats", "/stats", null],
   ["exam", "/exam", null],
   ["settings", "/", "settings"],
+  ["notfound", "/no-such-page", null],
 ];
 
 for (const [label, path, mode] of screens) {