Add loading, success and error feedback to passkey prompt tile

[rzueger]

Summary

The post-login passkey tile on the start page previously had three UX gaps during the registration flow:

1. No loading feedback: button disabled on click but no spinner — user unsure if anything was happening.
2. No success confirmation: on successful registration the tile silently disappeared — user could wonder whether it worked.
3. No error display: on failure the button simply re-enabled with no explanation.

All three are now addressed.

Changes

• Loading: replaces the custom styled button with the shared <Button> component using loading={submitting} — same spinner pattern as PasskeyLoginButton (login page) and PasskeyManager (profile page).
• Success: on successful registration the card replaces its content with a brief confirmation — "Eingerichtet! Beim nächsten Mal melden Sie sich ohne E-Mail-Code an." — shown for 3.5s before auto-dismissing. Matches the original tile promise ("ohne auf den E-Mail-Code zu warten").
• Failure: inline ErrorMessage below the button showing the existing profile.passkeysRegistrationFailure translation. Tile stays visible so the user can retry.

Test plan

• 1932 tests pass, typecheck clean
• Register a passkey via the tile on lszm-test
• Cancel the biometric — tile shows error
• Complete the biometric — tile shows success briefly, then dismisses
• Reload the page — tile no longer appears (user has a passkey)