refactor: extract YouTube download to ytmusic extension and fix UI issues

Remove built-in YouTube/Cobalt download pipeline from Go backend and
Dart frontend. YouTube downloading now requires the ytmusic-spotiflac
extension (with download_provider capability).

Go backend:
- Delete youtube.go (745 lines) and youtube_quality_test.go
- Remove DownloadFromYouTube, IsYouTubeURLExport,
  ExtractYouTubeVideoIDExport from exports.go
- Remove YouTube routing from DownloadTrack and DownloadByStrategy

Dart frontend:
- Remove YouTube from built-in services, bitrate settings, quality UI
- Remove youtubeOpusBitrate/youtubeMp3Bitrate from settings model
- Add migration 7: default service youtube -> tidal
- Remove YouTube l10n keys from all 14 arb files and regenerate
- Update _determineOutputExt to handle opus_/mp3_ quality strings
- Add SAF opus/mp3 metadata embedding in unified branch
- Fix TweenSequence assertion crash (t outside 0.0-1.0)
- Fix store URL TextField styling consistency

Extension changes (gitignored, in extension/YT-Music-SpotiFLAC/):
- Add download_provider type, qualityOptions, network permissions
- Implement checkAvailability and download via SpotubeDL/Cobalt

lib/models/settings.g.dart

@@ -47,8 +47,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   locale: json['locale'] as String? ?? 'system',
   lyricsMode: json['lyricsMode'] as String? ?? 'embed',
   tidalHighFormat: json['tidalHighFormat'] as String? ?? 'mp3_320',
-  youtubeOpusBitrate: (json['youtubeOpusBitrate'] as num?)?.toInt() ?? 256,
-  youtubeMp3Bitrate: (json['youtubeMp3Bitrate'] as num?)?.toInt() ?? 320,
   useAllFilesAccess: json['useAllFilesAccess'] as bool? ?? false,
   autoExportFailedDownloads:
       json['autoExportFailedDownloads'] as bool? ?? false,
@@ -125,8 +123,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'locale': instance.locale,
   'lyricsMode': instance.lyricsMode,
   'tidalHighFormat': instance.tidalHighFormat,
-  'youtubeOpusBitrate': instance.youtubeOpusBitrate,
-  'youtubeMp3Bitrate': instance.youtubeMp3Bitrate,
   'useAllFilesAccess': instance.useAllFilesAccess,
   'autoExportFailedDownloads': instance.autoExportFailedDownloads,
   'downloadNetworkMode': instance.downloadNetworkMode,