Mark active thread visited on selection

- Dispatch `MARK_THREAD_VISITED` when `activeThreadId` changes in root route
- Add reducer handling to update a thread's `lastVisitedAt`
- Add reducer test coverage for selecting a thread updates visit time
cd9ef00636Julius Marminge committed on 2/19/2026, 8:11:02 AMparent5cf140b
3 files changedLine totals unavailable