Add a metered/cellular network detector so downloads honour a "WiFi
only" preference. Android reports network type via NetworkTypeMonitor;
Rust exposes it through download/network.rs and holds the queue pump when
on a metered connection, emitting a queue-wide waitingForNetwork event.
The frontend surfaces this via the networkType service and a
waitingForNetwork store flag.
TRACES: UR-053 | DR-074
