Passkey login for stronger account security

Add optional passkey authentication for PayRequest dashboard accounts. Passkeys provide fast, phishing-resistant login using Face ID, Touch ID, Windows Hello, or a hardware security key. Users should be able to register and manage multiple passkeys, while magic-link and social login remain available for account recovery. The first version should focus on payrequest.app; support across payrequest.me and merchant custom domains can be evaluated separately because passkeys are bound to a relying-party domain.