feat: export failed downloads to TXT file

- Add Export button in queue when there are failed downloads
- Add auto-export setting in Download Settings
- Export includes track name, artist, Spotify/Deezer URL, and error message
- Clear Failed action in snackbar after export

lib/models/settings.dart

@@ -34,6 +34,7 @@ class AppSettings {
   final String lyricsMode;
   final String tidalHighFormat; // Format for Tidal HIGH quality: 'mp3_320' or 'opus_128'
   final bool useAllFilesAccess; // Android 13+ only: enable MANAGE_EXTERNAL_STORAGE
+  final bool autoExportFailedDownloads; // Auto export failed downloads to TXT file
 
   const AppSettings({
     this.defaultService = 'tidal',
@@ -66,6 +67,7 @@ class AppSettings {
     this.lyricsMode = 'embed',
     this.tidalHighFormat = 'mp3_320',
     this.useAllFilesAccess = false,
+    this.autoExportFailedDownloads = false,
   });
 
   AppSettings copyWith({
@@ -100,6 +102,7 @@ class AppSettings {
     String? lyricsMode,
     String? tidalHighFormat,
     bool? useAllFilesAccess,
+    bool? autoExportFailedDownloads,
   }) {
     return AppSettings(
       defaultService: defaultService ?? this.defaultService,
@@ -132,6 +135,7 @@ class AppSettings {
       lyricsMode: lyricsMode ?? this.lyricsMode,
       tidalHighFormat: tidalHighFormat ?? this.tidalHighFormat,
       useAllFilesAccess: useAllFilesAccess ?? this.useAllFilesAccess,
+      autoExportFailedDownloads: autoExportFailedDownloads ?? this.autoExportFailedDownloads,
     );
   }
 

lib/models/settings.g.dart

@@ -39,6 +39,8 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   lyricsMode: json['lyricsMode'] as String? ?? 'embed',
   tidalHighFormat: json['tidalHighFormat'] as String? ?? 'mp3_320',
   useAllFilesAccess: json['useAllFilesAccess'] as bool? ?? false,
+  autoExportFailedDownloads:
+      json['autoExportFailedDownloads'] as bool? ?? false,
 );
 
 Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
@@ -73,4 +75,5 @@ Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
       'lyricsMode': instance.lyricsMode,
       'tidalHighFormat': instance.tidalHighFormat,
       'useAllFilesAccess': instance.useAllFilesAccess,
+      'autoExportFailedDownloads': instance.autoExportFailedDownloads,
     };