fix: correct track/disc defaults, forward extension metadata, and fix service ID display

- Default track/disc number to 0 (unknown) instead of 1, letting the
  backend use the service-provided value or skip the field entirely
- Add releaseDate to ExploreItem so explore downloads carry release info
- Pass discNumber and releaseDate from extension album/playlist tracks
- Fix isDeezer detection using service field instead of substring match
- Add _displayServiceTrackId() to properly strip prefixes for all services

lib/services/download_request_payload.dart

@@ -53,8 +53,8 @@ class DownloadRequestPayload {
     this.artistTagMode = 'joined',
     this.embedLyrics = true,
     this.embedMaxQualityCover = true,
-    this.trackNumber = 1,
-    this.discNumber = 1,
+    this.trackNumber = 0,
+    this.discNumber = 0,
     this.totalTracks = 1,
     this.releaseDate = '',
     this.itemId = '',