Reset encryption keys #25

Provide a secure workflow for users to reset their encryption keys in case of device loss or compromise.

• In the security settings, add a “Reset encryption keys” button
• Require user re‑authentication (password/biometric)
• Display clear warnings: “All your previous messages will become unreadable to you. Your contacts will be notified.”
• After reset, generate new key pairs and re‑establish sessions with contacts
• Notify contacts that the user has reset keys (security alert)

This must be done carefully to avoid abuse or data loss.