Webauthn: Maybe don't have to ask the username

I come from pocket-id, where the webauthn flow is super conveniently just "press button, receive session". No email prompts. Homeauth requires the user name, which I think is a valid way to do it! But I really really love convenience, so I think there could be a better way.

My notes from reviewing the code in pocket-id are:

* Homeauth [uses](https://github.com/andrew-d/homeauth/blob/ddfbe3b7a4b9555d4eb2ae784249d9dc527749ec/cmd/homeauth/webauthn.go#L50) `s.webAuthn.BeginLogin` (which takes a username)
* Pocket-ID uses the "discoverable flow" and then uses the `ValidatePasskeyLogin` method to find the user that's associated with the offered passkey.

...is that a thing homeauth can do? Wants to do? I haven't read the webauthn or OIDC standards, but this seems like a valid way to handle stuff, right?