Closes AI-313

This PR makes sure an empty draft thread is reflected in the sidebar by
an empty thread item. The main goal of this change is to make the
existence of the drafts feature more intuitive and clear. We don't only
display the draft item now when moving away from it, but rather whenever
you're focused on it.

Release Notes:

- Agent: Improved the UX of the draft feature by always displaying a
corresponding thread item in the sidebar, even if the thread was still
empty.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>