feat: add 'By Playlist' folder organization option (#111)

lib/models/download_item.dart

@@ -34,6 +34,7 @@ class DownloadItem {
   final DownloadErrorType? errorType;
   final DateTime createdAt;
   final String? qualityOverride; // Override quality for this specific download
+  final String? playlistName; // Playlist context for folder organization
 
   const DownloadItem({
     required this.id,
@@ -48,6 +49,7 @@ class DownloadItem {
     this.errorType,
     required this.createdAt,
     this.qualityOverride,
+    this.playlistName,
   });
 
   DownloadItem copyWith({
@@ -63,6 +65,7 @@ class DownloadItem {
     DownloadErrorType? errorType,
     DateTime? createdAt,
     String? qualityOverride,
+    String? playlistName,
   }) {
     return DownloadItem(
       id: id ?? this.id,
@@ -77,6 +80,7 @@ class DownloadItem {
       errorType: errorType ?? this.errorType,
       createdAt: createdAt ?? this.createdAt,
       qualityOverride: qualityOverride ?? this.qualityOverride,
+      playlistName: playlistName ?? this.playlistName,
     );
   }
 

lib/models/download_item.g.dart

@@ -21,6 +21,7 @@ DownloadItem _$DownloadItemFromJson(Map<String, dynamic> json) => DownloadItem(
   errorType: $enumDecodeNullable(_$DownloadErrorTypeEnumMap, json['errorType']),
   createdAt: DateTime.parse(json['createdAt'] as String),
   qualityOverride: json['qualityOverride'] as String?,
+  playlistName: json['playlistName'] as String?,
 );
 
 Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
@@ -37,6 +38,7 @@ Map<String, dynamic> _$DownloadItemToJson(DownloadItem instance) =>
       'errorType': _$DownloadErrorTypeEnumMap[instance.errorType],
       'createdAt': instance.createdAt.toIso8601String(),
       'qualityOverride': instance.qualityOverride,
+      'playlistName': instance.playlistName,
     };
 
 const _$DownloadStatusEnumMap = {