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

ios/Runner/AppDelegate.swift

@@ -507,7 +507,8 @@ import Gobackend  // Import Go framework
         case "searchDeezerByISRC":
             let args = call.arguments as! [String: Any]
             let isrc = args["isrc"] as! String
-            let response = GobackendSearchDeezerByISRC(isrc, &error)
+            let itemId = args["item_id"] as? String ?? ""
+            let response = GobackendSearchDeezerByISRCForItemID(isrc, itemId, &error)
             if let error = error { throw error }
             return response