feat: add createPlaylistFolder setting for playlist source folder prefix

When enabled, playlist downloads are placed inside a subfolder named
after the playlist before the normal folder organization structure
(e.g. Playlist/<artist>/<album>/). The setting is a no-op when folder
organization is already set to 'By Playlist'. Includes model field,
JSON serialization, settings notifier, download queue path logic,
UI toggle in download settings, and localizations for all 13 languages.

lib/models/settings.dart

@@ -20,6 +20,7 @@ class AppSettings {
   final String updateChannel;
   final bool hasSearchedBefore;
   final String folderOrganization;
+  final bool createPlaylistFolder;
   final bool useAlbumArtistForFolders;
   final bool usePrimaryArtistOnly; // Strip featured artists from folder name
   final bool filterContributingArtistsInAlbumArtist;
@@ -96,6 +97,7 @@ class AppSettings {
     this.updateChannel = 'stable',
     this.hasSearchedBefore = false,
     this.folderOrganization = 'none',
+    this.createPlaylistFolder = false,
     this.useAlbumArtistForFolders = true,
     this.usePrimaryArtistOnly = false,
     this.filterContributingArtistsInAlbumArtist = false,
@@ -159,6 +161,7 @@ class AppSettings {
     String? updateChannel,
     bool? hasSearchedBefore,
     String? folderOrganization,
+    bool? createPlaylistFolder,
     bool? useAlbumArtistForFolders,
     bool? usePrimaryArtistOnly,
     bool? filterContributingArtistsInAlbumArtist,
@@ -215,6 +218,7 @@ class AppSettings {
       updateChannel: updateChannel ?? this.updateChannel,
       hasSearchedBefore: hasSearchedBefore ?? this.hasSearchedBefore,
       folderOrganization: folderOrganization ?? this.folderOrganization,
+      createPlaylistFolder: createPlaylistFolder ?? this.createPlaylistFolder,
       useAlbumArtistForFolders:
           useAlbumArtistForFolders ?? this.useAlbumArtistForFolders,
       usePrimaryArtistOnly: usePrimaryArtistOnly ?? this.usePrimaryArtistOnly,
@@ -255,8 +259,7 @@ class AppSettings {
       localLibraryBookmark: localLibraryBookmark ?? this.localLibraryBookmark,
       localLibraryShowDuplicates:
           localLibraryShowDuplicates ?? this.localLibraryShowDuplicates,
-      localLibraryAutoScan:
-          localLibraryAutoScan ?? this.localLibraryAutoScan,
+      localLibraryAutoScan: localLibraryAutoScan ?? this.localLibraryAutoScan,
       hasCompletedTutorial: hasCompletedTutorial ?? this.hasCompletedTutorial,
       lyricsProviders: lyricsProviders ?? this.lyricsProviders,
       lyricsIncludeTranslationNetease:

lib/models/settings.g.dart

@@ -23,6 +23,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   updateChannel: json['updateChannel'] as String? ?? 'stable',
   hasSearchedBefore: json['hasSearchedBefore'] as bool? ?? false,
   folderOrganization: json['folderOrganization'] as String? ?? 'none',
+  createPlaylistFolder: json['createPlaylistFolder'] as bool? ?? false,
   useAlbumArtistForFolders: json['useAlbumArtistForFolders'] as bool? ?? true,
   usePrimaryArtistOnly: json['usePrimaryArtistOnly'] as bool? ?? false,
   filterContributingArtistsInAlbumArtist:
@@ -100,6 +101,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'updateChannel': instance.updateChannel,
   'hasSearchedBefore': instance.hasSearchedBefore,
   'folderOrganization': instance.folderOrganization,
+  'createPlaylistFolder': instance.createPlaylistFolder,
   'useAlbumArtistForFolders': instance.useAlbumArtistForFolders,
   'usePrimaryArtistOnly': instance.usePrimaryArtistOnly,
   'filterContributingArtistsInAlbumArtist':