feat: add experimental Android native download worker

Introduce a service-owned download worker that offloads the full
download-and-finalize pipeline to DownloadService on Android, keeping
downloads alive independently of the Flutter UI process.

Key changes:
- Extract SAF download logic from MainActivity into SafDownloadHandler
- Add NativeDownloadFinalizer for Kotlin-side decryption, format
  conversion, metadata embedding, ReplayGain, post-processing, and
  history persistence
- Extend DownloadService with native queue management (start, pause,
  resume, cancel) using coroutine-based worker with AtomicFile snapshots
- Add Dart-side orchestration: snapshot polling, run-id correlation,
  adoption on app restart, and fallback to Dart queue
- Forward embedReplayGain, tidalHighFormat, and postProcessingEnabled
  through Go backend DownloadRequest struct
- Add nativeDownloadWorkerEnabled setting with UI toggle
- Make DownloadQueueLookup collections unmodifiable

lib/models/settings.dart

@@ -58,6 +58,8 @@ class AppSettings {
   networkCompatibilityMode; // Try HTTP + allow invalid TLS cert for API requests
   final String
   songLinkRegion; // SongLink userCountry region code used for platform lookup
+  final bool
+  nativeDownloadWorkerEnabled; // Experimental Android service-owned worker
 
   final bool localLibraryEnabled; // Enable local library scanning
   final String localLibraryPath; // Path to scan for audio files
@@ -133,6 +135,7 @@ class AppSettings {
     this.downloadNetworkMode = 'any',
     this.networkCompatibilityMode = false,
     this.songLinkRegion = 'US',
+    this.nativeDownloadWorkerEnabled = false,
     this.localLibraryEnabled = false,
     this.localLibraryPath = '',
     this.localLibraryBookmark = '',
@@ -196,6 +199,7 @@ class AppSettings {
     String? downloadNetworkMode,
     bool? networkCompatibilityMode,
     String? songLinkRegion,
+    bool? nativeDownloadWorkerEnabled,
     bool? localLibraryEnabled,
     String? localLibraryPath,
     String? localLibraryBookmark,
@@ -269,6 +273,8 @@ class AppSettings {
       networkCompatibilityMode:
           networkCompatibilityMode ?? this.networkCompatibilityMode,
       songLinkRegion: songLinkRegion ?? this.songLinkRegion,
+      nativeDownloadWorkerEnabled:
+          nativeDownloadWorkerEnabled ?? this.nativeDownloadWorkerEnabled,
       localLibraryEnabled: localLibraryEnabled ?? this.localLibraryEnabled,
       localLibraryPath: localLibraryPath ?? this.localLibraryPath,
       localLibraryBookmark: localLibraryBookmark ?? this.localLibraryBookmark,

lib/models/settings.g.dart

@@ -58,6 +58,8 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   downloadNetworkMode: json['downloadNetworkMode'] as String? ?? 'any',
   networkCompatibilityMode: json['networkCompatibilityMode'] as bool? ?? false,
   songLinkRegion: json['songLinkRegion'] as String? ?? 'US',
+  nativeDownloadWorkerEnabled:
+      json['nativeDownloadWorkerEnabled'] as bool? ?? false,
   localLibraryEnabled: json['localLibraryEnabled'] as bool? ?? false,
   localLibraryPath: json['localLibraryPath'] as String? ?? '',
   localLibraryBookmark: json['localLibraryBookmark'] as String? ?? '',
@@ -129,6 +131,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'downloadNetworkMode': instance.downloadNetworkMode,
   'networkCompatibilityMode': instance.networkCompatibilityMode,
   'songLinkRegion': instance.songLinkRegion,
+  'nativeDownloadWorkerEnabled': instance.nativeDownloadWorkerEnabled,
   'localLibraryEnabled': instance.localLibraryEnabled,
   'localLibraryPath': instance.localLibraryPath,
   'localLibraryBookmark': instance.localLibraryBookmark,