Honor sidebarThreadSortOrder in Sidebar V2 as an opt-in

[Mnigos]

I keep losing threads in the sidebar because active threads sort by creation time, so the thread I worked on five minutes ago can sit below ones from yesterday. I get why V2 keeps rows stable (#8231 makes a good case against reshuffling on every message), but for my workflow recency matters more than stability.

There's already a sidebarThreadSortOrder setting that defaults to updated_at and V2 just ignores it. Would you take a PR that wires that existing setting through Sidebar V2 as an opt-in, keeping the current stable order as the default? Happy to build it if the direction sounds right.