Add Mark unread thread action to sidebar context menu

- Add `mark-unread` option to thread context menu and dispatch reducer action
- Implement `MARK_THREAD_UNREAD` to set `lastVisitedAt` just before latest completion
- Add reducer tests for unread behavior and no-op when no completed turn
- Update fallback context menu styling so only delete uses destructive color
c0e3c1a93aJulius Marminge committed on 2/27/2026, 2:57:40 AMparent10c1cae
4 files changedLine totals unavailable