feat: remove Tidal built-in provider, add extension download dedup/ISRC/Lyrics APIs, and expand l10n/a11y

Remove Tidal from built-in provider registry (metadata, search, download,
URL parsing) and delete tidal.go. Introduce extension runtime APIs for
lyrics lookup (getLyricsLRC), ISRC existence check (checkISRCExists), and
ISRC index management (addToISRCIndex). Refactor extension download response
construction into normalizeExtensionDownloadResult/overlayExtensionDownloadMetadata
helpers with AlreadyExists support and ISRC indexing. Switch download mirrors
to DoRequestWithUserAgent for ISP blocking detection. Add 50+ new
localization keys and accessibility labels across all supported locales.

lib/widgets/download_service_picker.dart

@@ -21,8 +21,8 @@ class BuiltInService {
   });
 }
 
-const _builtInServices = [
-  BuiltInService(
+const _builtInServiceCatalog = {
+  'tidal': BuiltInService(
     id: 'tidal',
     label: 'Tidal',
     qualityOptions: [
@@ -43,7 +43,7 @@ const _builtInServices = [
       ),
     ],
   ),
-  BuiltInService(
+  'qobuz': BuiltInService(
     id: 'qobuz',
     label: 'Qobuz',
     qualityOptions: [
@@ -64,7 +64,7 @@ const _builtInServices = [
       ),
     ],
   ),
-];
+};
 
 class DownloadServicePicker extends ConsumerStatefulWidget {
   final String? trackName;
@@ -118,58 +118,93 @@ class DownloadServicePicker extends ConsumerStatefulWidget {
 class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
   late String _selectedService;
 
+  List<BuiltInService> _availableBuiltInServices() {
+    final availableIds = builtInDownloadProviderIds.toSet();
+    final services = <BuiltInService>[];
+    for (final id in availableIds) {
+      final service = _builtInServiceCatalog[id];
+      if (service != null) {
+        services.add(service);
+      }
+    }
+    return services;
+  }
+
+  List<Extension> _downloadExtensions() {
+    final extensionState = ref.read(extensionProvider);
+    return extensionState.extensions
+        .where((ext) => ext.enabled && ext.hasDownloadProvider)
+        .toList(growable: false);
+  }
+
+  bool _serviceExists(
+    String serviceId,
+    List<BuiltInService> builtInServices,
+    List<Extension> downloadExtensions,
+  ) {
+    if (serviceId.isEmpty) return false;
+    if (builtInServices.any((service) => service.id == serviceId)) return true;
+    return downloadExtensions.any((ext) => ext.id == serviceId);
+  }
+
   @override
   void initState() {
     super.initState();
+    final builtInServices = _availableBuiltInServices();
+    final downloadExtensions = _downloadExtensions();
     final recommended = widget.recommendedService;
-    if (recommended != null && recommended.isNotEmpty) {
+    if (recommended != null &&
+        _serviceExists(recommended, builtInServices, downloadExtensions)) {
       _selectedService = recommended;
     } else {
       _selectedService = ref.read(settingsProvider).defaultService;
     }
-    if (!_builtInServices.any((service) => service.id == _selectedService)) {
-      final extensionState = ref.read(extensionProvider);
-      final hasMatchingExtension = extensionState.extensions.any(
-        (ext) =>
-            ext.enabled &&
-            ext.hasDownloadProvider &&
-            ext.id == _selectedService,
-      );
-      if (!hasMatchingExtension) {
-        _selectedService = 'tidal';
-      }
+    if (!_serviceExists(
+      _selectedService,
+      builtInServices,
+      downloadExtensions,
+    )) {
+      _selectedService = builtInServices.isNotEmpty
+          ? builtInServices.first.id
+          : downloadExtensions.isNotEmpty
+          ? downloadExtensions.first.id
+          : '';
     }
   }
 
-  List<QualityOption> _getQualityOptions() {
-    final builtIn = _builtInServices
-        .where((s) => s.id == _selectedService)
+  List<QualityOption> _getQualityOptions(
+    List<BuiltInService> builtInServices,
+    List<Extension> downloadExtensions,
+  ) {
+    final builtIn = builtInServices
+        .where((service) => service.id == _selectedService)
         .firstOrNull;
     if (builtIn != null) {
       return builtIn.qualityOptions;
     }
 
-    final extensionState = ref.read(extensionProvider);
-    final ext = extensionState.extensions
+    final ext = downloadExtensions
         .where((e) => e.id == _selectedService)
         .firstOrNull;
     if (ext != null && ext.qualityOptions.isNotEmpty) {
       return ext.qualityOptions;
     }
 
-    return _builtInServices.firstWhere((s) => s.id == 'tidal').qualityOptions;
+    return const [];
   }
 
   @override
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
-    final extensionState = ref.watch(extensionProvider);
-
-    final downloadExtensions = extensionState.extensions
-        .where((ext) => ext.enabled && ext.hasDownloadProvider)
-        .toList();
-
-    final qualityOptions = _getQualityOptions();
+    ref.watch(extensionProvider);
+    final builtInServices = _availableBuiltInServices();
+    final downloadExtensions = _downloadExtensions();
+    final hasProviders =
+        builtInServices.isNotEmpty || downloadExtensions.isNotEmpty;
+    final qualityOptions = _getQualityOptions(
+      builtInServices,
+      downloadExtensions,
+    );
 
     return SafeArea(
       child: SingleChildScrollView(
@@ -213,68 +248,77 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
 
             Padding(
               padding: const EdgeInsets.symmetric(horizontal: 20),
-              child: Wrap(
-                spacing: 8,
-                runSpacing: 8,
-                children: [
-                  for (final service in _builtInServices)
-                    _ServiceChip(
-                      label: service.isDisabled
-                          ? '${service.label} (${service.disabledReason})'
-                          : widget.recommendedService == service.id
-                          ? '${service.label} (Recommended)'
-                          : service.label,
-                      isSelected: _selectedService == service.id,
-                      isDisabled: service.isDisabled,
-                      onTap: service.isDisabled
-                          ? null
-                          : () => setState(() => _selectedService = service.id),
+              child: hasProviders
+                  ? Wrap(
+                      spacing: 8,
+                      runSpacing: 8,
+                      children: [
+                        for (final service in builtInServices)
+                          _ServiceChip(
+                            label: service.isDisabled
+                                ? '${service.label} (${service.disabledReason})'
+                                : widget.recommendedService == service.id
+                                ? '${service.label} (Recommended)'
+                                : service.label,
+                            isSelected: _selectedService == service.id,
+                            isDisabled: service.isDisabled,
+                            onTap: service.isDisabled
+                                ? null
+                                : () => setState(
+                                    () => _selectedService = service.id,
+                                  ),
+                          ),
+                        for (final ext in downloadExtensions)
+                          _ServiceChip(
+                            label: widget.recommendedService == ext.id
+                                ? '${ext.displayName} (Recommended)'
+                                : ext.displayName,
+                            isSelected: _selectedService == ext.id,
+                            onTap: () =>
+                                setState(() => _selectedService = ext.id),
+                            iconPath: ext.iconPath,
+                          ),
+                      ],
+                    )
+                  : _NoDownloadProviderHint(
+                      primaryText: context.l10n.extensionsNoDownloadProvider,
+                      secondaryText: context.l10n.storeAddRepoDescription,
                     ),
-                  for (final ext in downloadExtensions)
-                    _ServiceChip(
-                      label: widget.recommendedService == ext.id
-                          ? '${ext.displayName} (Recommended)'
-                          : ext.displayName,
-                      isSelected: _selectedService == ext.id,
-                      onTap: () => setState(() => _selectedService = ext.id),
-                      iconPath: ext.iconPath,
-                    ),
-                ],
-              ),
             ),
-
-            Padding(
-              padding: const EdgeInsets.fromLTRB(24, 16, 24, 8),
-              child: Text(
-                context.l10n.downloadSelectQuality,
-                style: Theme.of(
-                  context,
-                ).textTheme.titleMedium?.copyWith(fontWeight: FontWeight.bold),
-              ),
-            ),
-
-            if (_builtInServices.any((s) => s.id == _selectedService))
+            if (hasProviders) ...[
               Padding(
-                padding: const EdgeInsets.fromLTRB(24, 0, 24, 12),
+                padding: const EdgeInsets.fromLTRB(24, 16, 24, 8),
                 child: Text(
-                  context.l10n.qualityNote,
-                  style: Theme.of(context).textTheme.bodySmall?.copyWith(
-                    color: colorScheme.onSurfaceVariant,
-                    fontStyle: FontStyle.italic,
+                  context.l10n.downloadSelectQuality,
+                  style: Theme.of(context).textTheme.titleMedium?.copyWith(
+                    fontWeight: FontWeight.bold,
                   ),
                 ),
               ),
-
-            for (final quality in qualityOptions)
-              _QualityOption(
-                title: quality.label,
-                subtitle: quality.description ?? '',
-                icon: _getQualityIcon(quality.id),
-                onTap: () {
-                  Navigator.pop(context);
-                  widget.onSelect(quality.id, _selectedService);
-                },
-              ),
+              if (builtInServices.any(
+                (service) => service.id == _selectedService,
+              ))
+                Padding(
+                  padding: const EdgeInsets.fromLTRB(24, 0, 24, 12),
+                  child: Text(
+                    context.l10n.qualityNote,
+                    style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                      color: colorScheme.onSurfaceVariant,
+                      fontStyle: FontStyle.italic,
+                    ),
+                  ),
+                ),
+              for (final quality in qualityOptions)
+                _QualityOption(
+                  title: _localizedQualityLabel(context, quality),
+                  subtitle: _localizedQualityDescription(context, quality),
+                  icon: _getQualityIcon(quality.id),
+                  onTap: () {
+                    Navigator.pop(context);
+                    widget.onSelect(quality.id, _selectedService);
+                  },
+                ),
+            ],
 
             const SizedBox(height: 16),
           ],
@@ -303,6 +347,35 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
         return Icons.music_note;
     }
   }
+
+  String _localizedQualityLabel(BuildContext context, QualityOption quality) {
+    switch (quality.id.toUpperCase()) {
+      case 'LOSSLESS':
+        return context.l10n.qualityFlacLossless;
+      case 'HI_RES':
+        return context.l10n.qualityHiResFlac;
+      case 'HI_RES_LOSSLESS':
+        return context.l10n.qualityHiResFlacMax;
+      default:
+        return quality.label;
+    }
+  }
+
+  String _localizedQualityDescription(
+    BuildContext context,
+    QualityOption quality,
+  ) {
+    switch (quality.id.toUpperCase()) {
+      case 'LOSSLESS':
+        return context.l10n.qualityFlacLosslessSubtitle;
+      case 'HI_RES':
+        return context.l10n.qualityHiResFlacSubtitle;
+      case 'HI_RES_LOSSLESS':
+        return context.l10n.qualityHiResFlacMaxSubtitle;
+      default:
+        return quality.description ?? '';
+    }
+  }
 }
 
 class _QualityOption extends StatelessWidget {
@@ -421,6 +494,63 @@ class _ServiceChip extends StatelessWidget {
   }
 }
 
+class _NoDownloadProviderHint extends StatelessWidget {
+  final String primaryText;
+  final String secondaryText;
+
+  const _NoDownloadProviderHint({
+    required this.primaryText,
+    required this.secondaryText,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
+    return Container(
+      width: double.infinity,
+      padding: const EdgeInsets.all(16),
+      decoration: BoxDecoration(
+        color: colorScheme.surfaceContainerHighest.withValues(alpha: 0.45),
+        borderRadius: BorderRadius.circular(16),
+        border: Border.all(
+          color: colorScheme.outlineVariant.withValues(alpha: 0.4),
+        ),
+      ),
+      child: Row(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Icon(
+            Icons.extension_outlined,
+            size: 18,
+            color: colorScheme.onSurfaceVariant,
+          ),
+          const SizedBox(width: 10),
+          Expanded(
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                Text(
+                  primaryText,
+                  style: Theme.of(
+                    context,
+                  ).textTheme.bodyMedium?.copyWith(fontWeight: FontWeight.w600),
+                ),
+                const SizedBox(height: 4),
+                Text(
+                  secondaryText,
+                  style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                    color: colorScheme.onSurfaceVariant,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
 class _TrackInfoHeader extends StatefulWidget {
   final String trackName;
   final String? artistName;