Update chat page with new navigation components

[bracesproul]

We need to add the following top level nav/actions to the chat page.

These should live outside of the shared deep agents package

• Agent name dropdown: render the same agent selection dropdown. when you switch agents it should update the query params, and remove all other query params to ensure you start fresh
• add the history button (lets reuse the history sidebar pop-out that we had before)
• add the inbox button. this should route to the /inbox page with the proper query params for the selected agent and thread ID. if a thread has not been started yet, we should disable this button
• have the settings icon open the edit agent dialog for now
• new thread should start a new thread