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

test/models_and_utils_test.dart

@@ -202,7 +202,6 @@ void main() {
 
       final updated = settings.copyWith(
         defaultService: 'tidal',
-        concurrentDownloads: 4,
         embedReplayGain: true,
         lyricsProviders: ['apple_music'],
         lyricsAppleElrcWordSync: true,
@@ -213,7 +212,6 @@ void main() {
       );
 
       expect(updated.defaultService, 'tidal');
-      expect(updated.concurrentDownloads, 4);
       expect(updated.embedReplayGain, isTrue);
       expect(updated.lyricsProviders, ['apple_music']);
       expect(updated.lyricsAppleElrcWordSync, isTrue);