Idle-poll audit caught a regression I introduced in the prior commit:
the topbar badge poll iterated chatApi.listMessages per thread on each
60s tick, firing N requests per refresh. Landing scene showed 9 reqs
in a 30s idle window (failed the <=5 budget).
Now the topbar badge just shows the THREAD COUNT (not per-thread
unread), which is a single chatApi.listThreads call. Per-thread unread
math stays in the Chat scene where it belongs (already wired:
chat-thread-row.unread + .chat-unread-summary).