fix(cache): send Cache-Control: no-store on all JSON/404 responses

Prevents a 404 (e.g. /manifest.json fetched before deploy) from being cached on
a device and persisting after the file exists — the cause of the manifest 404
on mobile but not desktop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/lib.js

@@ -2,7 +2,9 @@
 const now = () => Date.now();
 
 const json = (res, code, body) => {
-  res.writeHead(code, { 'Content-Type': 'application/json' });
+  // no-store: API/JSON responses (and 404s) must never be cached — a cached 404 for an asset
+  // like /manifest.json would otherwise persist on a device even after the file is deployed.
+  res.writeHead(code, { 'Content-Type': 'application/json', 'Cache-Control': 'no-store' });
   res.end(JSON.stringify(body));
 };