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/lib/stores/downloads.ts

@@ -40,7 +40,16 @@ export interface DownloadInfo {
 }
 
 export interface DownloadEvent {
-	type: 'queued' | 'started' | 'progress' | 'completed' | 'failed' | 'paused' | 'cancelled';
+	type:
+		| 'queued'
+		| 'started'
+		| 'progress'
+		| 'completed'
+		| 'failed'
+		| 'paused'
+		| 'cancelled'
+		| 'waitingForNetwork';
+	/** Absent on 'waitingForNetwork', which is queue-wide rather than per-download. */
 	downloadId: number;
 	itemId: string;
 	bytesDownloaded?: number;
@@ -64,6 +73,15 @@ interface DownloadsState {
 	stats: DownloadStats;
 }
 
+/**
+ * True when the download queue is held because "WiFi Only" is enabled and the
+ * device is on a metered/cellular network. Pending rows stay pending; the queue
+ * resumes automatically when an acceptable network appears.
+ *
+ * TRACES: UR-053 | DR-074
+ */
+export const waitingForNetwork = writable(false);
+
 function createDownloadsStore() {
 	const { subscribe, update, set } = writable<DownloadsState>({
 		downloads: {},
@@ -607,6 +625,17 @@ function handleDownloadEvent(payload: DownloadEvent): void {
 		case 'cancelled':
 			removeDownloadFromStore(payload.downloadId);
 			break;
+
+		case 'waitingForNetwork':
+			// Queue-wide, not tied to one download: the pump refused to start
+			// anything because WiFi-only is on and we're on a metered network.
+			waitingForNetwork.set(true);
+			break;
+	}
+
+	// Any per-download progress proves the gate isn't holding us any more.
+	if (payload.type === 'started' || payload.type === 'progress') {
+		waitingForNetwork.set(false);
 	}
 }