feat: propagate download cancellation through entire pipeline, add MusicBrainz album artist fallback, and allow disabling home feed

- Add reference-counted cancel entries to prevent premature cleanup when multiple operations share the same itemID
- Propagate cancellation to DownloadTrack, DownloadWithFallback, DownloadWithExtensionsJSON, extension providers, and ISRC search
- Fetch album artist from MusicBrainz when missing during download and re-enrich
- Make ALBUMARTIST tag nullable to avoid writing artistName as album artist
- Add home feed 'Off' option in extension settings
- Skip deezer in download provider priority sanitization

lib/services/platform_bridge.dart

@@ -609,9 +609,13 @@ class PlatformBridge {
     return jsonDecode(result as String) as Map<String, dynamic>;
   }
 
-  static Future<Map<String, dynamic>> searchDeezerByISRC(String isrc) async {
+  static Future<Map<String, dynamic>> searchDeezerByISRC(
+    String isrc, {
+    String? itemId,
+  }) async {
     final result = await _channel.invokeMethod('searchDeezerByISRC', {
       'isrc': isrc,
+      'item_id': itemId ?? '',
     });
     return jsonDecode(result as String) as Map<String, dynamic>;
   }