StatusRule now renders `{sinceLastMsg}/{sinceSession}` (e.g. `12s/3m 45s`)
when a user has submitted in the current session; falls back to the total
alone otherwise. Wires `lastUserAt` through the state/session lifecycle:
- useSubmission stamps `setLastUserAt(Date.now())` on send
- useSessionLifecycle nulls it in reset/resetVisibleHistory
- /branch slash nulls it on fork