Reset handshake

If someone loses the key, they can currently solve this problem instead by making a new contact for that person. That isn't so nice because you lose the history.

There could be a case where your contact has lost their device or accidentally deleted their encryption key. A "reset handshake" function would send the user back to the handshake screen, and then the new key would replace the old contact's key.

This is a stretch goal because it is actually a not straightforward to implement & test well, because it breaks the typical user flow in the UI and will require custom logic.

Start by locating the handshake screen and the existing flow for creating a new contact after a key is lost. Implement and test a reset-handshake path that preserves the contact's history while replacing its old encryption key with the new one.

Written by the indexing model from the issue text.