You are on the Express backend (this port). There is no page at / by default — opening this URL in a browser used to show "Cannot GET /".
The gallery / Patreon connect UI is the Next.js app: run npm run dev in the web/ folder (default http://localhost:3000).
NEXT_PUBLIC_RELAY_API_URL in web/.env.local should point here (e.g. http://127.0.0.1:8787) with no trailing slash.