Passwordless and security-key login UX, including the button label

## What

passwordless / security-key login UX: RK-less passwordless, username-less WebAuthn-only flow, and a configurable (not hardcoded) passwordless button label.

## Want

- Passwordless / security-key login UX: RK-less passwordless, username-less WebAuthn-only flow,
- A configurable (not hardcoded) passwordless button label

## Ideas this came from

- [#6901 Add flow options for RK-less passwordless login](https://github.com/goauthentik/authentik/issues/6901) — I'd like better flow options to properly implement passwordless login without having to use resident keys, so I can replicate my previous Keycloak setup. I'd li
- [#12475 Passwordless login Flow without having to type username and password only WebAuth](https://github.com/goauthentik/authentik/issues/12475) — Hello, Currently login with WebAuth requires typing your username and password It's not always convenient. It would be nice to have an option to authenticate us
- [#22042 stages/identification: passwordless flow button label is hardcoded to "Use a security key"](https://github.com/goauthentik/authentik/issues/22042) — IdentificationStage.passwordlessflow is a generic mechanism — admins point it at any authentication flow. But the button text is hardcoded to "Use a security ke

Start by reviewing the stages/identification passwordless-flow behavior and the related issues #6901, #12475, and #22042. Clarify how RK-less passwordless and username-less WebAuthn-only flows should work, then define how the passwordless button label becomes configurable; done means all requested UX cases and label behavior are supported.

Written by the indexing model from the issue text.