fix(ui): fetch covers via the Go backend instead of raw HttpClient

Both call sites used a bare dart:io HttpClient with no timeout, retry, or
cache; downloadCoverToFile provides all three (plus the singleflight cover
cache) and can hang-proof the embed path.

lib/providers/download_queue_provider_embedding.dart

@@ -903,22 +903,18 @@ extension _DownloadQueueEmbedding on DownloadQueueNotifier {
           '${DateTime.now().millisecondsSinceEpoch}_${Random().nextInt(10000)}';
       final coverPath = '${tempDir.path}/cover_embed_$uniqueId.jpg';
 
-      final httpClient = HttpClient();
-      try {
-        final request = await httpClient.getUrl(Uri.parse(coverUrl));
-        final response = await request.close();
-        if (response.statusCode != 200) {
-          _log.w('Failed to download cover: HTTP ${response.statusCode}');
-          return null;
-        }
-        final sink = File(coverPath).openWrite();
-        await response.pipe(sink);
-        await sink.close();
-        _log.d('Cover downloaded for embedding: $coverPath');
-        return coverPath;
-      } finally {
-        httpClient.close();
+      // Go's cover pipeline: shared cache/singleflight, retries, timeouts.
+      final result = await PlatformBridge.downloadCoverToFile(
+        coverUrl,
+        coverPath,
+        maxQuality: false,
+      );
+      if (result['error'] != null) {
+        _log.w('Failed to download cover: ${result['error']}');
+        return null;
       }
+      _log.d('Cover downloaded for embedding: $coverPath');
+      return coverPath;
     } catch (e) {
       _log.e('Failed to download cover for embedding: $e');
       return null;

lib/screens/track_metadata_edit_sheet.dart

@@ -938,12 +938,12 @@ class _EditMetadataSheetState extends State<_EditMetadataSheet> {
             );
             final coverOutput =
                 '${tempDir.path}${Platform.pathSeparator}cover.jpg';
-            final response = await HttpClient()
-                .getUrl(Uri.parse(coverUrl))
-                .then((req) => req.close());
+            await PlatformBridge.downloadCoverToFile(
+              coverUrl,
+              coverOutput,
+              maxQuality: false,
+            );
             final file = File(coverOutput);
-            final sink = file.openWrite();
-            await response.pipe(sink);
             if (await file.exists() && await file.length() > 0) {
               await _cleanupSelectedCoverTemp();
               if (mounted) {