fix(ios): use security-scoped bookmarks for download directory persistence

- Switch iOS bookmark creation from .minimalBookmark to .withSecurityScope
- Add .withSecurityScope option when resolving bookmarks
- Add downloadDirectoryBookmark field to AppSettings for persisting iOS bookmarks
- Resolve bookmark and startAccessingIosBookmark before queue processing
- Guarantee stopAccessingIosBookmark cleanup via try/finally
- Create bookmark on folder pick in both setup screen and download settings
- Clear bookmark when switching to SAF mode or iOS path normalization
- Fix stale bottom sheet context usage (ctx -> context) in download settings

lib/models/settings.dart

@@ -11,6 +11,7 @@ class AppSettings {
   final String audioQuality;
   final String filenameFormat;
   final String downloadDirectory;
+  final String downloadDirectoryBookmark;
   final String storageMode; // 'app' or 'saf'
   final String downloadTreeUri; // SAF persistable tree URI
   final bool autoFallback;
@@ -89,6 +90,7 @@ class AppSettings {
     this.audioQuality = 'LOSSLESS',
     this.filenameFormat = '{title} - {artist}',
     this.downloadDirectory = '',
+    this.downloadDirectoryBookmark = '',
     this.storageMode = 'app',
     this.downloadTreeUri = '',
     this.autoFallback = true,
@@ -153,6 +155,7 @@ class AppSettings {
     String? audioQuality,
     String? filenameFormat,
     String? downloadDirectory,
+    String? downloadDirectoryBookmark,
     String? storageMode,
     String? downloadTreeUri,
     bool? autoFallback,
@@ -213,6 +216,8 @@ class AppSettings {
       audioQuality: audioQuality ?? this.audioQuality,
       filenameFormat: filenameFormat ?? this.filenameFormat,
       downloadDirectory: downloadDirectory ?? this.downloadDirectory,
+      downloadDirectoryBookmark:
+          downloadDirectoryBookmark ?? this.downloadDirectoryBookmark,
       storageMode: storageMode ?? this.storageMode,
       downloadTreeUri: downloadTreeUri ?? this.downloadTreeUri,
       autoFallback: autoFallback ?? this.autoFallback,

lib/models/settings.g.dart

@@ -11,6 +11,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   audioQuality: json['audioQuality'] as String? ?? 'LOSSLESS',
   filenameFormat: json['filenameFormat'] as String? ?? '{title} - {artist}',
   downloadDirectory: json['downloadDirectory'] as String? ?? '',
+  downloadDirectoryBookmark: json['downloadDirectoryBookmark'] as String? ?? '',
   storageMode: json['storageMode'] as String? ?? 'app',
   downloadTreeUri: json['downloadTreeUri'] as String? ?? '',
   autoFallback: json['autoFallback'] as bool? ?? true,
@@ -86,6 +87,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'audioQuality': instance.audioQuality,
   'filenameFormat': instance.filenameFormat,
   'downloadDirectory': instance.downloadDirectory,
+  'downloadDirectoryBookmark': instance.downloadDirectoryBookmark,
   'storageMode': instance.storageMode,
   'downloadTreeUri': instance.downloadTreeUri,
   'autoFallback': instance.autoFallback,