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.dart

@@ -42,10 +42,6 @@ class AppSettings {
   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: 128/256/320 kbps)
-  final int
-  youtubeMp3Bitrate; // YouTube MP3 bitrate (supported: 128/256/320 kbps)
   final bool
   useAllFilesAccess; // Android 13+ only: enable MANAGE_EXTERNAL_STORAGE
   final bool
@@ -121,8 +117,6 @@ class AppSettings {
     this.locale = 'system',
     this.lyricsMode = 'embed',
     this.tidalHighFormat = 'mp3_320',
-    this.youtubeOpusBitrate = 256,
-    this.youtubeMp3Bitrate = 320,
     this.useAllFilesAccess = false,
     this.autoExportFailedDownloads = false,
     this.downloadNetworkMode = 'any',
@@ -189,8 +183,6 @@ class AppSettings {
     String? locale,
     String? lyricsMode,
     String? tidalHighFormat,
-    int? youtubeOpusBitrate,
-    int? youtubeMp3Bitrate,
     bool? useAllFilesAccess,
     bool? autoExportFailedDownloads,
     String? downloadNetworkMode,
@@ -257,8 +249,6 @@ class AppSettings {
       locale: locale ?? this.locale,
       lyricsMode: lyricsMode ?? this.lyricsMode,
       tidalHighFormat: tidalHighFormat ?? this.tidalHighFormat,
-      youtubeOpusBitrate: youtubeOpusBitrate ?? this.youtubeOpusBitrate,
-      youtubeMp3Bitrate: youtubeMp3Bitrate ?? this.youtubeMp3Bitrate,
       useAllFilesAccess: useAllFilesAccess ?? this.useAllFilesAccess,
       autoExportFailedDownloads:
           autoExportFailedDownloads ?? this.autoExportFailedDownloads,