refactor: remove built-in provider registry in favor of extensions

All search, metadata, and download providers are now exclusively
supplied by extensions. The built-in provider registry that previously
exposed Deezer/Tidal/Qobuz as hardcoded providers is fully removed.

Removed across Go, Dart, Kotlin, and Swift:
- BuiltInProviderSpec class, registry, and all accessor helpers
- SearchProviderAllJSON, GetBuiltInProvidersJSON, ParseProviderURLJSON,
  ParseDeezerURLExport Go exports and their platform channel bindings
- Built-in provider items in search dropdown, service picker, and
  provider priority UI lists
- provider_ui_utils.dart helper file

Deezer metadata enrichment (ISRC lookup, extended metadata, cover
upgrade) remains fully functional through direct DeezerClient calls
in the download pipeline — these are not part of the provider
registry and are unaffected.

Mark deezer as a retired built-in metadata provider so stale user
priority lists are cleaned up on next launch.

lib/services/platform_bridge.dart

@@ -884,23 +884,6 @@ class PlatformBridge {
     await _channel.invokeMethod('clearTrackCache');
   }
 
-  static Future<Map<String, dynamic>> searchProviderAll(
-    String providerId,
-    String query, {
-    int trackLimit = 15,
-    int artistLimit = 2,
-    String? filter,
-  }) async {
-    final result = await _channel.invokeMethod('searchProviderAll', {
-      'provider_id': providerId,
-      'query': query,
-      'track_limit': trackLimit,
-      'artist_limit': artistLimit,
-      'filter': filter ?? '',
-    });
-    return _decodeRequiredMapResult(result, 'searchProviderAll');
-  }
-
   static Future<Map<String, dynamic>> getDeezerRelatedArtists(
     String artistId, {
     int limit = 12,
@@ -912,13 +895,6 @@ class PlatformBridge {
     return _decodeRequiredMapResult(result, 'getDeezerRelatedArtists');
   }
 
-  static Future<Map<String, dynamic>> parseProviderUrl(String url) async {
-    final result = await _channel.invokeMethod('parseProviderUrl', {
-      'url': url,
-    });
-    return _decodeRequiredMapResult(result, 'parseProviderUrl');
-  }
-
   static Future<Map<String, dynamic>> getProviderMetadata(
     String providerId,
     String resourceType,
@@ -1394,11 +1370,6 @@ class PlatformBridge {
     return _decodeMapListResult(result, 'getSearchProviders');
   }
 
-  static Future<List<Map<String, dynamic>>> getBuiltInProviders() async {
-    final result = await _channel.invokeMethod('getBuiltInProviders');
-    return _decodeMapListResult(result, 'getBuiltInProviders');
-  }
-
   static Future<Map<String, dynamic>?> handleURLWithExtension(
     String url,
   ) async {