fix(ios): local library scan fails on iOS due to missing security-scoped bookmark access

lib/models/settings.dart

@@ -57,6 +57,8 @@ class AppSettings {
 
   final bool localLibraryEnabled; // Enable local library scanning
   final String localLibraryPath; // Path to scan for audio files
+  final String
+  localLibraryBookmark; // Base64-encoded iOS security-scoped bookmark
   final bool
   localLibraryShowDuplicates; // Show indicator when searching for existing tracks
 
@@ -122,6 +124,7 @@ class AppSettings {
     this.songLinkRegion = 'US',
     this.localLibraryEnabled = false,
     this.localLibraryPath = '',
+    this.localLibraryBookmark = '',
     this.localLibraryShowDuplicates = true,
     this.hasCompletedTutorial = false,
     this.lyricsProviders = const [
@@ -185,6 +188,7 @@ class AppSettings {
     String? songLinkRegion,
     bool? localLibraryEnabled,
     String? localLibraryPath,
+    String? localLibraryBookmark,
     bool? localLibraryShowDuplicates,
     bool? hasCompletedTutorial,
     List<String>? lyricsProviders,
@@ -249,6 +253,7 @@ class AppSettings {
       songLinkRegion: songLinkRegion ?? this.songLinkRegion,
       localLibraryEnabled: localLibraryEnabled ?? this.localLibraryEnabled,
       localLibraryPath: localLibraryPath ?? this.localLibraryPath,
+      localLibraryBookmark: localLibraryBookmark ?? this.localLibraryBookmark,
       localLibraryShowDuplicates:
           localLibraryShowDuplicates ?? this.localLibraryShowDuplicates,
       hasCompletedTutorial: hasCompletedTutorial ?? this.hasCompletedTutorial,

lib/models/settings.g.dart

@@ -55,6 +55,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   songLinkRegion: json['songLinkRegion'] as String? ?? 'US',
   localLibraryEnabled: json['localLibraryEnabled'] as bool? ?? false,
   localLibraryPath: json['localLibraryPath'] as String? ?? '',
+  localLibraryBookmark: json['localLibraryBookmark'] as String? ?? '',
   localLibraryShowDuplicates:
       json['localLibraryShowDuplicates'] as bool? ?? true,
   hasCompletedTutorial: json['hasCompletedTutorial'] as bool? ?? false,
@@ -128,6 +129,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'songLinkRegion': instance.songLinkRegion,
   'localLibraryEnabled': instance.localLibraryEnabled,
   'localLibraryPath': instance.localLibraryPath,
+  'localLibraryBookmark': instance.localLibraryBookmark,
   'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,
   'hasCompletedTutorial': instance.hasCompletedTutorial,
   'lyricsProviders': instance.lyricsProviders,