feat: add external player mode for local library playback

lib/models/settings.dart

@@ -12,6 +12,7 @@ class AppSettings {
   final String downloadTreeUri; // SAF persistable tree URI
   final bool autoFallback;
   final bool autoSkipUnavailableTracks;
+  final String playerMode; // 'internal' or 'external'
   final bool smartQueueEnabled; // Enable smart curated autoplay queue
   final bool embedMetadata; // Master switch for metadata/cover/lyrics embedding
   final bool embedLyrics;
@@ -92,6 +93,7 @@ class AppSettings {
     this.downloadTreeUri = '',
     this.autoFallback = true,
     this.autoSkipUnavailableTracks = true,
+    this.playerMode = 'internal',
     this.smartQueueEnabled = true,
     this.embedMetadata = true,
     this.embedLyrics = true,
@@ -160,6 +162,7 @@ class AppSettings {
     String? downloadTreeUri,
     bool? autoFallback,
     bool? autoSkipUnavailableTracks,
+    String? playerMode,
     bool? smartQueueEnabled,
     bool? embedMetadata,
     bool? embedLyrics,
@@ -222,6 +225,7 @@ class AppSettings {
       autoFallback: autoFallback ?? this.autoFallback,
       autoSkipUnavailableTracks:
           autoSkipUnavailableTracks ?? this.autoSkipUnavailableTracks,
+      playerMode: playerMode ?? this.playerMode,
       smartQueueEnabled: smartQueueEnabled ?? this.smartQueueEnabled,
       embedMetadata: embedMetadata ?? this.embedMetadata,
       embedLyrics: embedLyrics ?? this.embedLyrics,

lib/models/settings.g.dart

@@ -15,6 +15,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   downloadTreeUri: json['downloadTreeUri'] as String? ?? '',
   autoFallback: json['autoFallback'] as bool? ?? true,
   autoSkipUnavailableTracks: json['autoSkipUnavailableTracks'] as bool? ?? true,
+  playerMode: json['playerMode'] as String? ?? 'internal',
   smartQueueEnabled: json['smartQueueEnabled'] as bool? ?? true,
   embedMetadata: json['embedMetadata'] as bool? ?? true,
   embedLyrics: json['embedLyrics'] as bool? ?? true,
@@ -93,6 +94,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'downloadTreeUri': instance.downloadTreeUri,
   'autoFallback': instance.autoFallback,
   'autoSkipUnavailableTracks': instance.autoSkipUnavailableTracks,
+  'playerMode': instance.playerMode,
   'smartQueueEnabled': instance.smartQueueEnabled,
   'embedMetadata': instance.embedMetadata,
   'embedLyrics': instance.embedLyrics,