feat: add extension system with skipBuiltInFallback support

- Add extension manager, manifest, runtime, providers, settings
- Add extension provider and UI pages (extensions, detail, priority)
- Add download service picker widget
- Add metadata provider priority page
- Add source field to Track model for extension tracking
- Add skipBuiltInFallback manifest option to skip built-in providers
- Update download queue to use source extension first
- Add extension upgrade support without data loss

lib/models/settings.dart

@@ -24,6 +24,8 @@ class AppSettings {
   final bool useCustomSpotifyCredentials; // Whether to use custom credentials (if set)
   final String metadataSource; // spotify, deezer - source for search and metadata
   final bool enableLogging; // Enable detailed logging for debugging
+  final bool useExtensionProviders; // Use extension providers for downloads when available
+  final String? searchProvider; // null/empty = default (Deezer/Spotify), otherwise extension ID
 
   const AppSettings({
     this.defaultService = 'tidal',
@@ -46,6 +48,8 @@ class AppSettings {
     this.useCustomSpotifyCredentials = true, // Default: use custom if set
     this.metadataSource = 'deezer', // Default: Deezer (no rate limit)
     this.enableLogging = false, // Default: disabled for performance
+    this.useExtensionProviders = true, // Default: use extensions when available
+    this.searchProvider, // Default: null (use Deezer/Spotify)
   });
 
   AppSettings copyWith({
@@ -69,6 +73,8 @@ class AppSettings {
     bool? useCustomSpotifyCredentials,
     String? metadataSource,
     bool? enableLogging,
+    bool? useExtensionProviders,
+    String? searchProvider,
   }) {
     return AppSettings(
       defaultService: defaultService ?? this.defaultService,
@@ -91,6 +97,8 @@ class AppSettings {
       useCustomSpotifyCredentials: useCustomSpotifyCredentials ?? this.useCustomSpotifyCredentials,
       metadataSource: metadataSource ?? this.metadataSource,
       enableLogging: enableLogging ?? this.enableLogging,
+      useExtensionProviders: useExtensionProviders ?? this.useExtensionProviders,
+      searchProvider: searchProvider ?? this.searchProvider,
     );
   }
 

lib/models/settings.g.dart

@@ -28,6 +28,8 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['useCustomSpotifyCredentials'] as bool? ?? true,
   metadataSource: json['metadataSource'] as String? ?? 'deezer',
   enableLogging: json['enableLogging'] as bool? ?? false,
+  useExtensionProviders: json['useExtensionProviders'] as bool? ?? true,
+  searchProvider: json['searchProvider'] as String?,
 );
 
 Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
@@ -52,4 +54,6 @@ Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
       'useCustomSpotifyCredentials': instance.useCustomSpotifyCredentials,
       'metadataSource': instance.metadataSource,
       'enableLogging': instance.enableLogging,
+      'useExtensionProviders': instance.useExtensionProviders,
+      'searchProvider': instance.searchProvider,
     };

lib/models/track.dart

@@ -18,6 +18,7 @@ class Track {
   final String? releaseDate;
   final String? deezerId;
   final ServiceAvailability? availability;
+  final String? source; // Extension ID that provided this track (null for built-in sources)
 
   const Track({
     required this.id,
@@ -33,10 +34,14 @@ class Track {
     this.releaseDate,
     this.deezerId,
     this.availability,
+    this.source,
   });
 
   factory Track.fromJson(Map<String, dynamic> json) => _$TrackFromJson(json);
   Map<String, dynamic> toJson() => _$TrackToJson(this);
+  
+  /// Check if this track is from an extension
+  bool get isFromExtension => source != null && source!.isNotEmpty;
 }
 
 @JsonSerializable()

lib/models/track.g.dart

@@ -24,6 +24,7 @@ Track _$TrackFromJson(Map<String, dynamic> json) => Track(
       : ServiceAvailability.fromJson(
           json['availability'] as Map<String, dynamic>,
         ),
+  source: json['source'] as String?,
 );
 
 Map<String, dynamic> _$TrackToJson(Track instance) => <String, dynamic>{
@@ -40,6 +41,7 @@ Map<String, dynamic> _$TrackToJson(Track instance) => <String, dynamic>{
   'releaseDate': instance.releaseDate,
   'deezerId': instance.deezerId,
   'availability': instance.availability,
+  'source': instance.source,
 };
 
 ServiceAvailability _$ServiceAvailabilityFromJson(Map<String, dynamic> json) =>