Allow to pause work on a session

When I go home I need to close the application and would want a pause button so I don't wait 20 minutes for a session to complete and then have to close it anyway and loose the work that was done in those 20 minutes.

The issue names no files, tests, or entry points. First clarify the session pause and resume behavior, including how work is preserved when the application closes, then identify the relevant session lifecycle code and tests. Done means a session can be paused before closing and resumed without losing completed work.

Written by the indexing model from the issue text.