feat: primary artist only folders, fix notifications v20, fix SAF duplicate dirs

- Add 'Use Primary Artist Only' setting to strip featured artists from folder names
- Fix flutter_local_notifications v20 breaking changes (positional params)
- Fix SAF duplicate folder bug: synchronized ensureDocumentDir to prevent race condition creating empty folders with (1), (2) suffixes during concurrent downloads

lib/models/settings.dart

@@ -20,6 +20,7 @@ class AppSettings {
   final bool hasSearchedBefore;
   final String folderOrganization;
   final bool useAlbumArtistForFolders;
+  final bool usePrimaryArtistOnly; // Strip featured artists from folder name
   final String historyViewMode;
   final String historyFilterMode;
   final bool askQualityBeforeDownload;
@@ -65,6 +66,7 @@ class AppSettings {
     this.hasSearchedBefore = false,
     this.folderOrganization = 'none',
     this.useAlbumArtistForFolders = true,
+    this.usePrimaryArtistOnly = false,
     this.historyViewMode = 'grid',
     this.historyFilterMode = 'all',
     this.askQualityBeforeDownload = true,
@@ -109,6 +111,7 @@ class AppSettings {
     bool? hasSearchedBefore,
     String? folderOrganization,
     bool? useAlbumArtistForFolders,
+    bool? usePrimaryArtistOnly,
     String? historyViewMode,
     String? historyFilterMode,
     bool? askQualityBeforeDownload,
@@ -154,6 +157,8 @@ class AppSettings {
       folderOrganization: folderOrganization ?? this.folderOrganization,
       useAlbumArtistForFolders:
           useAlbumArtistForFolders ?? this.useAlbumArtistForFolders,
+      usePrimaryArtistOnly:
+          usePrimaryArtistOnly ?? this.usePrimaryArtistOnly,
       historyViewMode: historyViewMode ?? this.historyViewMode,
       historyFilterMode: historyFilterMode ?? this.historyFilterMode,
       askQualityBeforeDownload: askQualityBeforeDownload ?? this.askQualityBeforeDownload,

lib/models/settings.g.dart

@@ -23,6 +23,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   hasSearchedBefore: json['hasSearchedBefore'] as bool? ?? false,
   folderOrganization: json['folderOrganization'] as String? ?? 'none',
   useAlbumArtistForFolders: json['useAlbumArtistForFolders'] as bool? ?? true,
+  usePrimaryArtistOnly: json['usePrimaryArtistOnly'] as bool? ?? false,
   historyViewMode: json['historyViewMode'] as String? ?? 'grid',
   historyFilterMode: json['historyFilterMode'] as String? ?? 'all',
   askQualityBeforeDownload: json['askQualityBeforeDownload'] as bool? ?? true,
@@ -70,6 +71,7 @@ Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
       'hasSearchedBefore': instance.hasSearchedBefore,
       'folderOrganization': instance.folderOrganization,
       'useAlbumArtistForFolders': instance.useAlbumArtistForFolders,
+      'usePrimaryArtistOnly': instance.usePrimaryArtistOnly,
       'historyViewMode': instance.historyViewMode,
       'historyFilterMode': instance.historyFilterMode,
       'askQualityBeforeDownload': instance.askQualityBeforeDownload,