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.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,