feat: add Opus 320kbps quality, remove Tidal HIGH tier

- Add YouTubeQualityOpus320 constant and opus_320 parser case in Go backend
- Expand opus supported bitrates to [128, 256, 320] across Go, Dart settings, and UI
- Update default YouTube Opus option from 256 to 320kbps
- Remove Tidal HIGH (lossy 320kbps) quality from Go backend, settings model,
  settings provider, download queue provider (both SAF and non-SAF paths),
  settings UI (quality option, format picker, helper methods), and l10n keys
- Add settings migration v6: auto-migrate users with audioQuality=HIGH to LOSSLESS
- Update and add Go test cases for opus_320 and adjusted max bitrate
- Regenerate l10n files, remove 10 unused downloadLossy* l10n keys

lib/models/settings.dart

@@ -38,10 +38,8 @@ class AppSettings {
   final bool showExtensionStore;
   final String locale;
   final String lyricsMode;
-  final String
-  tidalHighFormat; // Format for Tidal HIGH quality: 'mp3_320', 'opus_256', or 'opus_128'
   final int
-  youtubeOpusBitrate; // YouTube Opus bitrate (supported: 128/256 kbps)
+  youtubeOpusBitrate; // YouTube Opus bitrate (supported: 128/256/320 kbps)
   final int
   youtubeMp3Bitrate; // YouTube MP3 bitrate (supported: 128/256/320 kbps)
   final bool
@@ -114,7 +112,6 @@ class AppSettings {
     this.showExtensionStore = true,
     this.locale = 'system',
     this.lyricsMode = 'embed',
-    this.tidalHighFormat = 'mp3_320',
     this.youtubeOpusBitrate = 256,
     this.youtubeMp3Bitrate = 320,
     this.useAllFilesAccess = false,
@@ -178,7 +175,6 @@ class AppSettings {
     bool? showExtensionStore,
     String? locale,
     String? lyricsMode,
-    String? tidalHighFormat,
     int? youtubeOpusBitrate,
     int? youtubeMp3Bitrate,
     bool? useAllFilesAccess,
@@ -241,7 +237,6 @@ class AppSettings {
       showExtensionStore: showExtensionStore ?? this.showExtensionStore,
       locale: locale ?? this.locale,
       lyricsMode: lyricsMode ?? this.lyricsMode,
-      tidalHighFormat: tidalHighFormat ?? this.tidalHighFormat,
       youtubeOpusBitrate: youtubeOpusBitrate ?? this.youtubeOpusBitrate,
       youtubeMp3Bitrate: youtubeMp3Bitrate ?? this.youtubeMp3Bitrate,
       useAllFilesAccess: useAllFilesAccess ?? this.useAllFilesAccess,

lib/models/settings.g.dart

@@ -44,7 +44,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   showExtensionStore: json['showExtensionStore'] as bool? ?? true,
   locale: json['locale'] as String? ?? 'system',
   lyricsMode: json['lyricsMode'] as String? ?? 'embed',
-  tidalHighFormat: json['tidalHighFormat'] as String? ?? 'mp3_320',
   youtubeOpusBitrate: (json['youtubeOpusBitrate'] as num?)?.toInt() ?? 256,
   youtubeMp3Bitrate: (json['youtubeMp3Bitrate'] as num?)?.toInt() ?? 320,
   useAllFilesAccess: json['useAllFilesAccess'] as bool? ?? false,
@@ -119,7 +118,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'showExtensionStore': instance.showExtensionStore,
   'locale': instance.locale,
   'lyricsMode': instance.lyricsMode,
-  'tidalHighFormat': instance.tidalHighFormat,
   'youtubeOpusBitrate': instance.youtubeOpusBitrate,
   'youtubeMp3Bitrate': instance.youtubeMp3Bitrate,
   'useAllFilesAccess': instance.useAllFilesAccess,