feat(downloads): support up to 3 concurrent downloads

The Dart queue loop was already built around an activeDownloads map and
per-item progress tracking but was gated to one download at a time. Add
a concurrentDownloads setting (1-3, default 1) exposed in Download
Settings > Performance, and let the scheduler keep up to that many
items in flight. The experimental Android native worker is still
strictly sequential, so it is skipped in favor of the Dart queue while
concurrency is enabled.

lib/models/settings.dart

@@ -62,6 +62,8 @@ class AppSettings {
   songLinkRegion; // SongLink userCountry region code used for platform lookup
   final bool
   nativeDownloadWorkerEnabled; // Experimental Android service-owned worker
+  final int
+  concurrentDownloads; // Max simultaneous downloads in the Dart queue (1-3)
 
   final bool localLibraryEnabled;
   final String localLibraryPath;
@@ -140,6 +142,7 @@ class AppSettings {
     this.allowLocalNetwork = false,
     this.songLinkRegion = 'US',
     this.nativeDownloadWorkerEnabled = false,
+    this.concurrentDownloads = 1,
     this.localLibraryEnabled = false,
     this.localLibraryPath = '',
     this.localLibraryBookmark = '',
@@ -208,6 +211,7 @@ class AppSettings {
     bool? allowLocalNetwork,
     String? songLinkRegion,
     bool? nativeDownloadWorkerEnabled,
+    int? concurrentDownloads,
     bool? localLibraryEnabled,
     String? localLibraryPath,
     String? localLibraryBookmark,
@@ -289,6 +293,7 @@ class AppSettings {
       songLinkRegion: songLinkRegion ?? this.songLinkRegion,
       nativeDownloadWorkerEnabled:
           nativeDownloadWorkerEnabled ?? this.nativeDownloadWorkerEnabled,
+      concurrentDownloads: concurrentDownloads ?? this.concurrentDownloads,
       localLibraryEnabled: localLibraryEnabled ?? this.localLibraryEnabled,
       localLibraryPath: localLibraryPath ?? this.localLibraryPath,
       localLibraryBookmark: localLibraryBookmark ?? this.localLibraryBookmark,

lib/models/settings.g.dart

@@ -62,6 +62,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   songLinkRegion: json['songLinkRegion'] as String? ?? 'US',
   nativeDownloadWorkerEnabled:
       json['nativeDownloadWorkerEnabled'] as bool? ?? false,
+  concurrentDownloads: (json['concurrentDownloads'] as num?)?.toInt() ?? 1,
   localLibraryEnabled: json['localLibraryEnabled'] as bool? ?? false,
   localLibraryPath: json['localLibraryPath'] as String? ?? '',
   localLibraryBookmark: json['localLibraryBookmark'] as String? ?? '',
@@ -138,6 +139,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'allowLocalNetwork': instance.allowLocalNetwork,
   'songLinkRegion': instance.songLinkRegion,
   'nativeDownloadWorkerEnabled': instance.nativeDownloadWorkerEnabled,
+  'concurrentDownloads': instance.concurrentDownloads,
   'localLibraryEnabled': instance.localLibraryEnabled,
   'localLibraryPath': instance.localLibraryPath,
   'localLibraryBookmark': instance.localLibraryBookmark,