Passkey deletion detection and rotation prompt

[praveenperera]

Detect when backup passkey has been deleted and prompt user to re-create.

• iOS: silent startup check using preferImmediatelyAvailableCredentials
• Android 14+: silent startup check using prepareGetCredential() — no biometric prompt
• Android <14: local credential ID cache with periodic verification (every 30 days, on backup settings open, after extended background)
• Warn user if passkey is missing, prompt to create a new one (rotation)
• Store credential ID, RP ID, and last-verified timestamp locally