feat(downloads): WiFi-only network-type-aware download gating

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

src-tauri/src/download/events.rs

@@ -41,6 +41,11 @@ pub enum DownloadEvent {
     /// Download cancelled
     #[serde(rename_all = "camelCase")]
     Cancelled { download_id: i64, item_id: String },
+    /// The queue is holding: WiFi-only is enabled and the current network is
+    /// metered/cellular. Pending rows stay pending and resume on network change.
+    ///
+    /// TRACES: UR-053 | DR-074
+    WaitingForNetwork,
 }
 
 #[cfg(test)]