This is very nice and simple. A few areas for improvement, in my opinion: the URL should be easy to copy, paste, or type into another device. I'd suggest designing the route like pindsend.app/pin/CODEHERE. Also, for some reason, copying and pasting the URL didn't seem to work in its current form. I would also consider implementing a QR code to allow quick scanning and redirection on another device, especially a phone, see Wormhole's implementation for reference [1]. Regarding security, the PIN looks quite short; I'd suggest increasing its complexity or length and automatically expiring sessions after a few minutes of inactivity. Additionally, definitely consider implementing end-to-end encryption if you haven’t already.
[1] https://wormhole.app