Context provider for the status bar. Holds the current message and exposes
setStatusText / flashStatusText (the latter auto-clears via a timer) to
descendants through useStatusBar.
Renders the bottom status bar showing the current message from useStatusBar, or nothing when there's no message.
Hook returning the status-bar context, used to read or update the message shown in the bottom status bar (e.g. hovered link URLs, transient flashes).