This page only talks to your PortCode, directly from this browser. GitHub Pages hosts the file; it never sees your key or your session. The key stays in this browser, and only a one-time proof of it goes over the connection.
Not set up yet? Run portcode serve --tunnel on your machine: it prints a link and a QR code that open this page already connected. Or read how to reach it safely (Tailscale, cloudflared or an SSH tunnel). Plain ws:// only works for 127.0.0.1; anything else must be wss://.