I would love to see the GUI support WebAuthn as an alternative to password authentication. Although the Syncthing GUI perhaps doesn't need the strongest security since it's typically not exposed to the whole internet, WebAuthn can still offer a much more pleasant user experience than passwords for those users that prefer it. Personally I use a password manager protected by a YubiKey, and it's rather annoying to have to take the detour through the password manager when I could just tap the YubiKey instead - especially when accessing the Syncthing GUI remotely on my phone, I wouldn't have to switch back and forth between apps to retrieve a password.

I have a working, fairly polished implementation ready, though I expect it'll take a bit more work for it to pass review (most notably, I haven't added any tests yet). Shall I open a pull request, or would you like to discuss it some more first?