feature request: Fork a conversation

It would be great to have a fork option for conversations.

Often I'm mid-chat and want to try a different direction without losing the original thread. Right now the only way is to manually copy everything into a new chat, which is tedious.

A simple "fork from here" that splits the conversation at a chosen message would do it.

Same ask as #1748, which was closed due to the v3 code freeze. v4 is out now, so raising it again as suggested there.