shouldShowWifiUnavailableBanner

Returns true when the "Wi-Fi unavailable" recovery banner should render in ConnectionsScreen.

Banner shows only while a network scan is actively running, local-network permission is granted, WiFi is unavailable, and the scan has not yet produced any discovered TCP nodes. The auto-scan case is covered because isNetworkScanning is true during auto-scan regardless of the user's active Connections pane.

Gating on the scan state keeps the banner silent while discovery is idle — the user only needs the recovery hint at the moment a scan cannot find a usable transport. The localNetworkPermissionGranted guard keeps the banner from overlapping the permission-request flow on the scan toggle.

The banner is a recovery hint for the case where the user has started a network scan but no nodes have been discovered yet. Once the scan produces results, the user has found what they were looking for and the hint is no longer useful — so the banner is suppressed when the discovered-TCP list is non-empty.