[UX - Duo Chat] Improve Navigation between Chat and Chat History

Problem

Recently we introduced a chat history for Duo chat for internal users to test. We've received a bit of feedback that the navigation to and from the history could be improved

• When I click on the history button for whatever reason, and then I realize I actually don't want to go to another thread, there is no way for me to get back to the current thread other than picking it from the list (which is likely the top entry but not necessarily). Maybe because I am the PM for this and am just playing with this feature, I am more likely go to the list and then wanting to go back than real users. #522957 (comment 2392229444)

We've also received feedback that the chat header moves, because the icons change depending on which screen you are on: #530584 (closed)

Proposals:

The Draft MR

Add a back button to the chat history view, that allows users to navigate back to their chat:

chat-back-to-chat.mov

This solution simplifies the interaction between chats and chat history, and also stops the header from jumping.