fix(download): suffix batch filenames with track number when needed

Mark multi-track queue additions as batch downloads and append
{track:02} - {title} when the configured format has no uniqueness
token, instead of relying on playlist position alone.

lib/models/download_item.dart

@@ -38,6 +38,7 @@ class DownloadItem {
   final String? qualityOverride;
   final String? playlistName;
   final int? playlistPosition; // 1-based position in the source playlist
+  final bool fromBatch;
 
   const DownloadItem({
     required this.id,
@@ -55,6 +56,7 @@ class DownloadItem {
     this.qualityOverride,
     this.playlistName,
     this.playlistPosition,
+    this.fromBatch = false,
   });
 
   DownloadItem copyWith({
@@ -73,6 +75,7 @@ class DownloadItem {
     String? qualityOverride,
     String? playlistName,
     int? playlistPosition,
+    bool? fromBatch,
   }) {
     return DownloadItem(
       id: id ?? this.id,
@@ -90,6 +93,7 @@ class DownloadItem {
       qualityOverride: qualityOverride ?? this.qualityOverride,
       playlistName: playlistName ?? this.playlistName,
       playlistPosition: playlistPosition ?? this.playlistPosition,
+      fromBatch: fromBatch ?? this.fromBatch,
     );
   }
 

lib/models/download_item.g.dart

@@ -24,6 +24,7 @@ DownloadItem _$DownloadItemFromJson(Map<String, dynamic> json) => DownloadItem(
   qualityOverride: json['qualityOverride'] as String?,
   playlistName: json['playlistName'] as String?,
   playlistPosition: (json['playlistPosition'] as num?)?.toInt(),
+  fromBatch: json['fromBatch'] as bool? ?? false,
 );
 
 Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
@@ -43,6 +44,7 @@ Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
       'qualityOverride': instance.qualityOverride,
       'playlistName': instance.playlistName,
       'playlistPosition': instance.playlistPosition,
+      'fromBatch': instance.fromBatch,
     };
 
 const _$DownloadStatusEnumMap = {