The sessions screen was using useEffect with [client] to load sessions,
which only fired when the client object reference changed. If the app
was backgrounded and later foregrounded (common scenario on mobile),
the session list was never refreshed, showing stale data.
Replaced with useFocusEffect from expo-router so sessions are
reloaded every time the sessions tab is focused — including:
- Initial mount
- Coming back from background
- Switching tabs and returning
