refactor(download): remove concurrent download option

The download API only permits one request at a time, so parallel
downloads are removed to avoid wasted/blocked API calls. Downloads
now always run sequentially (one track at a time).

- Drop concurrentDownloads from AppSettings + JSON serialization
- Remove setConcurrentDownloads and the settings UI (1-5 chips + warning)
- Strip optionsConcurrent* l10n keys from all ARBs and regenerate
- Rework queue worker into _processQueueSequential (single active download)
- Update marketing copy and adjust tests

lib/models/settings.g.dart

@@ -21,7 +21,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   maxQualityCover: json['maxQualityCover'] as bool? ?? true,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
-  concurrentDownloads: (json['concurrentDownloads'] as num?)?.toInt() ?? 1,
   checkForUpdates: json['checkForUpdates'] as bool? ?? true,
   updateChannel: json['updateChannel'] as String? ?? 'stable',
   hasSearchedBefore: json['hasSearchedBefore'] as bool? ?? false,
@@ -102,7 +101,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedReplayGain': instance.embedReplayGain,
   'maxQualityCover': instance.maxQualityCover,
   'isFirstLaunch': instance.isFirstLaunch,
-  'concurrentDownloads': instance.concurrentDownloads,
   'checkForUpdates': instance.checkForUpdates,
   'updateChannel': instance.updateChannel,
   'hasSearchedBefore': instance.hasSearchedBefore,