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

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -2946,8 +2946,9 @@ class MainActivity: FlutterFragmentActivity() {
                         }
                         "searchDeezerByISRC" -> {
                             val isrc = call.argument<String>("isrc") ?: ""
+                            val itemId = call.argument<String>("item_id") ?: ""
                             val response = withContext(Dispatchers.IO) {
-                                Gobackend.searchDeezerByISRC(isrc)
+                                Gobackend.searchDeezerByISRCForItemID(isrc, itemId)
                             }
                             result.success(response)
                         }