Email capture for accounts that are created with passkeys

### What is the issue?

At this time, it doesn't look like passkeys-created accounts have any kind of step in the sign up flow where email addresses are collected. That results in incorrect email addresses on the "Contact preferences" page:

### Steps to reproduce

- Create an account with a passkey.
- Visit contact preferences page: https://login.tailscale.com/admin/settings/contact-preferences

### Are there any recent changes that introduced the issue?

Passkeys announced on June 7, 2023:

https://tailscale.com/blog/passkeys/

### OS

_No response_

### OS version

_No response_

### Tailscale version

_No response_

### Other software

_No response_

### Bug report

_No response_

Start with the passkey account creation flow and the Contact preferences page at login.tailscale.com/admin/settings/contact-preferences, using the linked passkeys announcement for context. Reproduce the issue by creating an account with a passkey and inspect how its email address reaches Contact preferences. Done means passkey-created accounts collect and display the correct email address.

Written by the indexing model from the issue text.