fix(sw): stop precaching /404.html, it 403s and breaks the whole addAll

nginx serves /404.html as internal-only (see docker/nginx.conf) so it
preserves the real 404 status via error_page - any direct client fetch,
including the service worker's install-time addAll, gets a 403. Since
addAll is atomic, that one bad URL was failing precache of the entire
app shell.

public/sw.js

@@ -1,14 +1,13 @@
 // Bump this on every release that changes the app shell or the vendored
 // engine build — it's the only thing that invalidates old caches, since
 // none of the cached URLs below are content-hashed by us.
-const CACHE_VERSION = "v1";
+const CACHE_VERSION = "v2";
 const SHELL_CACHE = `shell-${CACHE_VERSION}`;
 const ENGINE_CACHE = `engine-${CACHE_VERSION}`;
 
 const SHELL_ASSETS = [
   "/",
   "/play/",
-  "/404.html",
   "/manifest.webmanifest",
   "/icons/icon-192.png",
   "/icons/icon-512.png",