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
0d67424f69Julius Marminge committed on 2/27/2026, 3:13:24 AM· committed by Julius Marmingeparent1245ce9
4 files changedLine totals unavailable