feat(tui): show time-since-last-user-message alongside session total (#8541)

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

ui-tui/src/components/appLayout.tsx

@@ -188,6 +188,7 @@ const ComposerPane = memo(function ComposerPane({
             busy={ui.busy}
             cols={composer.cols}
             cwdLabel={status.cwdLabel}
+            lastUserAt={status.lastUserAt}
             model={ui.info?.model?.split('/').pop() ?? ''}
             sessionStartedAt={status.sessionStartedAt}
             showCost={ui.showCost}