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/utils/clickable_metadata.dart

@@ -61,17 +61,6 @@ Future<List<Map<String, dynamic>>> _searchMetadataProvider(
   required String filter,
   required int limit,
 }) async {
-  if (isBuiltInSearchProvider(providerId)) {
-    final result = await PlatformBridge.searchProviderAll(
-      providerId,
-      query,
-      trackLimit: 0,
-      artistLimit: filter == 'artist' ? limit : 0,
-      filter: filter,
-    );
-    return _extractSearchItems(result, filter);
-  }
-
   return PlatformBridge.customSearchWithExtension(
     providerId,
     query,
@@ -79,24 +68,6 @@ Future<List<Map<String, dynamic>>> _searchMetadataProvider(
   );
 }
 
-List<Map<String, dynamic>> _extractSearchItems(
-  Map<String, dynamic> result,
-  String filter,
-) {
-  final key = switch (filter) {
-    'artist' => 'artists',
-    'album' => 'albums',
-    _ => '${filter}s',
-  };
-  final items = result[key];
-  if (items is! List) return const [];
-
-  return items
-      .whereType<Map<Object?, Object?>>()
-      .map((item) => Map<String, dynamic>.from(item))
-      .toList(growable: false);
-}
-
 List<String> _metadataSearchProviderCandidates(
   BuildContext context, {
   String? sourceProviderId,
@@ -142,10 +113,6 @@ List<String> _metadataSearchProviderCandidates(
     addProvider(extension.id);
   }
 
-  for (final providerId in builtInSearchProviderIds) {
-    addProvider(providerId);
-  }
-
   return candidates;
 }
 
@@ -153,7 +120,6 @@ bool _canSearchMetadataProvider(
   String providerId,
   ExtensionState extensionState,
 ) {
-  if (isBuiltInSearchProvider(providerId)) return true;
   return extensionState.extensions.any(
     (ext) => ext.enabled && ext.hasCustomSearch && ext.id == providerId,
   );
@@ -333,8 +299,7 @@ void _pushArtistScreen(
   String? coverUrl,
   String? extensionId,
 }) {
-  final isExtension =
-      extensionId != null && !isBuiltInMetadataProvider(extensionId);
+  final isExtension = extensionId != null;
   final resolvedProviderId = extensionId;
 
   _pushViaPreferredNavigator(
@@ -362,8 +327,7 @@ void _pushAlbumScreen(
   String? coverUrl,
   String? extensionId,
 }) {
-  final isExtension =
-      extensionId != null && !isBuiltInMetadataProvider(extensionId);
+  final isExtension = extensionId != null;
   final resolvedExtensionId = extensionId;
 
   _pushViaPreferredNavigator(
@@ -677,19 +641,9 @@ bool _canNavigateArtistDirectly({
   required String? extensionId,
 }) {
   if (extensionId != null) return true;
-  final providerPrefix = _resourceProviderPrefix(artistId);
-  if (providerPrefix != null && isBuiltInMetadataProvider(providerPrefix)) {
-    return true;
-  }
   return _spotifyArtistIdPattern.hasMatch(artistId);
 }
 
-String? _resourceProviderPrefix(String resourceId) {
-  final colonIndex = resourceId.indexOf(':');
-  if (colonIndex <= 0) return null;
-  return resourceId.substring(0, colonIndex).trim();
-}
-
 final RegExp _spotifyArtistIdPattern = RegExp(r'^[A-Za-z0-9]{22}$');
 
 class ClickableAlbumName extends StatelessWidget {

lib/utils/provider_ui_utils.dart

@@ -1,24 +0,0 @@
-import 'package:flutter/material.dart';
-import 'package:spotiflac_android/providers/extension_provider.dart';
-
-IconData resolveProviderIcon(
-  String providerId, {
-  IconData tidalIcon = Icons.music_note,
-  IconData builtInDefaultIcon = Icons.album,
-  IconData deezerIcon = Icons.graphic_eq,
-  IconData fallbackIcon = Icons.extension,
-}) {
-  final builtIn = builtInProviderSpecForId(providerId);
-  if (builtIn != null) {
-    if (providerId == 'tidal') {
-      return tidalIcon;
-    }
-    return builtInDefaultIcon;
-  }
-
-  if (providerId == 'deezer') {
-    return deezerIcon;
-  }
-
-  return fallbackIcon;
-}