Support passwordless sign-in with passkeys

### Describe the problem

WebAuthn currently works only as a second factor. Even with a passkey enrolled, signing in still requires typing e-mail and password first. A passkey with user verification is already two factors on its own, something you have plus a biometric or PIN, so the password on top adds little. I would like to sign in with the passkey alone, no password.

### Solution brainstorm

Make passwordless login configurable and add a passkey button to the sign-in page.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

I am happy to implement this.

Start by tracing the existing WebAuthn second-factor flow and the sign-in page. Determine where passwordless login can be made configurable and where a passkey button belongs. Done means a user with a verified passkey can sign in without entering an e-mail or password, while the existing password flow remains available.

Written by the indexing model from the issue text.