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/widgets/animation_utils.dart

@@ -748,7 +748,7 @@ class _DownloadSuccessOverlayState extends State<DownloadSuccessOverlay>
     _flashAnimation = TweenSequence<double>([
       TweenSequenceItem(tween: Tween(begin: 0.0, end: 0.15), weight: 30),
       TweenSequenceItem(tween: Tween(begin: 0.15, end: 0.0), weight: 70),
-    ]).animate(CurvedAnimation(parent: _controller, curve: Curves.easeOut));
+    ]).animate(_controller);
   }
 
   @override
@@ -816,7 +816,7 @@ class _AnimatedBadgeState extends State<AnimatedBadge>
     _scaleAnimation = TweenSequence<double>([
       TweenSequenceItem(tween: Tween(begin: 1.0, end: 1.3), weight: 40),
       TweenSequenceItem(tween: Tween(begin: 1.3, end: 1.0), weight: 60),
-    ]).animate(CurvedAnimation(parent: _controller, curve: Curves.easeOutBack));
+    ]).animate(_controller);
   }
 
   @override

lib/widgets/download_service_picker.dart

@@ -77,24 +77,6 @@ const _builtInServices = [
       ),
     ],
   ),
-  BuiltInService(
-    id: 'youtube',
-    label: 'YouTube',
-    qualityOptions: [
-      QualityOption(
-        id: 'opus_320',
-        label: 'Opus 320kbps',
-        description: 'Best quality lossy (~10MB per track)',
-      ),
-      QualityOption(
-        id: 'mp3_320',
-        label: 'MP3 320kbps',
-        description: 'Best compatibility (~10MB per track)',
-      ),
-    ],
-    isDisabled: false,
-    disabledReason: null,
-  ),
 ];
 
 class DownloadServicePicker extends ConsumerStatefulWidget {
@@ -148,9 +130,6 @@ class DownloadServicePicker extends ConsumerStatefulWidget {
 }
 
 class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
-  static const List<int> _youtubeOpusSupportedBitrates = [128, 256, 320];
-  static const List<int> _youtubeMp3SupportedBitrates = [128, 256, 320];
-
   late String _selectedService;
 
   @override
@@ -167,30 +146,6 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
 
   /// Get quality options for the selected service
   List<QualityOption> _getQualityOptions() {
-    final settings = ref.read(settingsProvider);
-    if (_selectedService == 'youtube') {
-      final opusBitrate = _nearestSupportedBitrate(
-        settings.youtubeOpusBitrate,
-        _youtubeOpusSupportedBitrates,
-      );
-      final mp3Bitrate = _nearestSupportedBitrate(
-        settings.youtubeMp3Bitrate,
-        _youtubeMp3SupportedBitrates,
-      );
-      return [
-        QualityOption(
-          id: 'opus_$opusBitrate',
-          label: 'Opus ${opusBitrate}kbps',
-          description: 'Configured from YouTube settings',
-        ),
-        QualityOption(
-          id: 'mp3_$mp3Bitrate',
-          label: 'MP3 ${mp3Bitrate}kbps',
-          description: 'Configured from YouTube settings',
-        ),
-      ];
-    }
-
     final builtIn = _builtInServices
         .where((s) => s.id == _selectedService)
         .firstOrNull;
@@ -215,22 +170,6 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
     ];
   }
 
-  int _nearestSupportedBitrate(int value, List<int> supported) {
-    var nearest = supported.first;
-    var nearestDistance = (value - nearest).abs();
-
-    for (final option in supported.skip(1)) {
-      final distance = (value - option).abs();
-      if (distance < nearestDistance ||
-          (distance == nearestDistance && option > nearest)) {
-        nearest = option;
-        nearestDistance = distance;
-      }
-    }
-
-    return nearest;
-  }
-
   @override
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
@@ -324,9 +263,7 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
               ),
             ),
 
-            if (_builtInServices.any(
-              (s) => s.id == _selectedService && s.id != 'youtube',
-            ))
+            if (_builtInServices.any((s) => s.id == _selectedService))
               Padding(
                 padding: const EdgeInsets.fromLTRB(24, 0, 24, 12),
                 child: Text(
@@ -338,18 +275,6 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
                 ),
               ),
 
-            if (_selectedService == 'youtube')
-              Padding(
-                padding: const EdgeInsets.fromLTRB(24, 0, 24, 12),
-                child: Text(
-                  context.l10n.youtubeQualityNote,
-                  style: Theme.of(context).textTheme.bodySmall?.copyWith(
-                    color: colorScheme.onSurfaceVariant,
-                    fontStyle: FontStyle.italic,
-                  ),
-                ),
-              ),
-
             for (final quality in qualityOptions)
               _QualityOption(
                 title: quality.label,