Sidebar: cap the thread list above "Working" with scroll, keep "Working" pinned

[lamat1111]

Problem

When there are many threads above the "Working" section in the left sidebar, the list grows and pushes the "Working" section down, out of view. To check a running thread I have to scroll the whole sidebar.

Proposal

• Cap the number of visible rows in the thread list above "Working" (a sensible default, ideally configurable).
• Past that cap, that list becomes its own scrollable area.
• The "Working" section stays pinned at the bottom of the sidebar, always visible.

Related

#27137 describes the same pain from the Cmd+K side (working threads are too far down). This idea fixes it in the sidebar itself. I am also proposing a highlight for the "Working" header row in a separate idea.