fix: disable Vercel Next.js framework detection

Add "framework": null to vercel.json so Vercel doesn't try to
run Next.js build flow (looking for .next/routes-manifest.json).

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

vercel.json

@@ -1,4 +1,5 @@
 {
+  "framework": null,
   "bunVersion": "1.x",
   "rewrites": [
     { "source": "/(.*)", "destination": "/api/relay" }