Add 2FA methods (TOTP first, then passkeys)

[taivlam]

Currently, there are no 2FA methods when logging into LinkStack.

The first method that could be implemented is TOTP. This would mean Aegis as a local Android app or Ente Auth for a cloud E2EE cross platform app (or any other option that is "Google Authenticator"-compatible).

The second method that could be implemented is passkeys, which is available as a sign-in option on GitHub. Bitwarden is also offering logging into Bitwarden accounts via passkeys.

Not sure if both methods (TOTP and passkeys) should be split from each other, though I hope LinkStack implements these 2FA methods for account sign-ins in the future.

[taivlam]

Never mind, just found #161.