Add webAuthn 2FA to support Yubikeys as additional 2nd factors

[tobiasgruber]

Added webAuthn as a second option for Two Factor Authentication to allow Yubikeys to be used as second factors.

• Claude Code helped me to refactor the logic to allow multiple second factor types.
• The second factor login screen only shows the 2fa variants that exist for the account.
• Tested locally with Yubikey 4 and 5.

Added migration Version20260519120000.php which must be executed after deployment.

• widens the secret column on sandstorm_neostwofactorauthentication_domain_model_secondfactor from VARCHAR(255) to TEXT, so it can hold the serialized WebAuthn credential data (which exceeds 255 chars). Without it, registering a security key will fail to persist.
• run it with ./flow doctrine:migrate

Added settings for webAuthn configuration - see Settings.yaml for example.

Screenshots:
Register 2nd Factor Select Screen:

Register Security Key Screen:

List of registered 2nd Factors Screen:

Second factor login Screen: