Require iOS 18.4 minimum deployment target

[praveenperera]

Lock down iOS minimum deployment target to 18.4 before shipping passkey cloud backup.

iOS 18.0-18.3 has a Cross-Device Authentication bug where PRF outputs vary depending on authentication method, which can cause data loss (same passkey produces different encryption keys). Requiring 18.4 avoids this entirely.

[praveenperera]

Closing as completed. The iOS deployment target is locked to 18.4 in the Xcode project.