UI should not allow user to amend groupby whilst in-flight

If user initiates a groupby on a large table, this may take a few seconds to complete. Do not allow the user to add additional elements to the groupby until server responds.

Because this will be limiting with existing UI, UI should offer a way to compose a multi-column groupBy clause , to be then submitted as a unit

The issue names no files or tests; start by locating the UI groupby control and the request lifecycle that waits for the server response. Confirm where in-flight changes are accepted, then define completion as preventing additions during that request while allowing multiple columns to be composed and submitted as one groupBy clause.

Written by the indexing model from the issue text.