refactor(ui): share legacy provider-id resource helpers

legacyProviderIdFromResourceId and stripPrefixedResourceId were
copy-pasted in the album, artist, and playlist screens with slightly
different null semantics; the shared nullable variant plus explicit
?? 'spotify' fallbacks makes the difference visible at the call sites.

lib/screens/album_screen.dart

@@ -14,6 +14,7 @@ import 'package:spotiflac_android/services/platform_bridge.dart';
 import 'package:spotiflac_android/utils/image_cache_utils.dart';
 import 'package:spotiflac_android/utils/string_utils.dart';
 import 'package:spotiflac_android/utils/nav_bar_inset.dart';
+import 'package:spotiflac_android/utils/provider_resource_ids.dart';
 import 'package:spotiflac_android/widgets/download_service_picker.dart';
 import 'package:spotiflac_android/widgets/animation_utils.dart';
 import 'package:spotiflac_android/providers/library_collections_provider.dart';
@@ -92,31 +93,17 @@ class _AlbumScreenState extends ConsumerState<AlbumScreen> {
   bool _tallHeader = false;
   final ScrollController _scrollController = ScrollController();
 
-  String _legacyProviderIdFromResourceId(String value) {
-    if (value.startsWith('deezer:')) return 'deezer';
-    if (value.startsWith('qobuz:')) return 'qobuz';
-    if (value.startsWith('tidal:')) return 'tidal';
-    if (value.startsWith('spotify:')) return 'spotify';
-    return 'spotify';
-  }
 
   String _effectiveMetadataProviderIdFromAlbumId() {
     if (widget.extensionId != null && widget.extensionId!.isNotEmpty) {
       return widget.extensionId!;
     }
     return resolveEffectiveMetadataProvider(
-      _legacyProviderIdFromResourceId(widget.albumId),
+      legacyProviderIdFromResourceId(widget.albumId) ?? 'spotify',
       ref.read(extensionProvider),
     );
   }
 
-  String _stripPrefixedResourceId(String value) {
-    final colonIndex = value.indexOf(':');
-    if (colonIndex <= 0 || colonIndex == value.length - 1) {
-      return value;
-    }
-    return value.substring(colonIndex + 1);
-  }
 
   @override
   void initState() {
@@ -336,7 +323,7 @@ class _AlbumScreenState extends ConsumerState<AlbumScreen> {
   }
 
   String _metadataResourceId(String providerId) {
-    return _stripPrefixedResourceId(widget.albumId);
+    return stripPrefixedResourceId(widget.albumId);
   }
 
   double _albumTitleFontSize() {

lib/screens/artist_screen.dart

@@ -14,6 +14,7 @@ import 'package:spotiflac_android/providers/local_library_provider.dart';
 import 'package:spotiflac_android/services/platform_bridge.dart';
 import 'package:spotiflac_android/utils/string_utils.dart';
 import 'package:spotiflac_android/utils/nav_bar_inset.dart';
+import 'package:spotiflac_android/utils/provider_resource_ids.dart';
 import 'package:spotiflac_android/screens/album_screen.dart';
 import 'package:spotiflac_android/screens/home_tab.dart'
     show ExtensionAlbumScreen;
@@ -168,31 +169,17 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen> {
     return _directMetadataProviderId();
   }
 
-  String _legacyProviderIdFromResourceId(String value) {
-    if (value.startsWith('deezer:')) return 'deezer';
-    if (value.startsWith('qobuz:')) return 'qobuz';
-    if (value.startsWith('tidal:')) return 'tidal';
-    if (value.startsWith('spotify:')) return 'spotify';
-    return 'spotify';
-  }
 
   String _effectiveMetadataProviderIdFromArtistId() {
     if (widget.extensionId != null && widget.extensionId!.isNotEmpty) {
       return widget.extensionId!;
     }
     return resolveEffectiveMetadataProvider(
-      _legacyProviderIdFromResourceId(widget.artistId),
+      legacyProviderIdFromResourceId(widget.artistId) ?? 'spotify',
       ref.read(extensionProvider),
     );
   }
 
-  String _stripPrefixedResourceId(String value) {
-    final colonIndex = value.indexOf(':');
-    if (colonIndex <= 0 || colonIndex == value.length - 1) {
-      return value;
-    }
-    return value.substring(colonIndex + 1);
-  }
 
   String? _directMetadataProviderId() {
     final providerId = _effectiveMetadataProviderIdFromArtistId();
@@ -200,7 +187,7 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen> {
   }
 
   String _metadataResourceId(String providerId) {
-    return _stripPrefixedResourceId(widget.artistId);
+    return stripPrefixedResourceId(widget.artistId);
   }
 
   @override
@@ -1104,7 +1091,7 @@ class _ArtistScreenState extends ConsumerState<ArtistScreen> {
   Future<List<Track>> _fetchAlbumTracks(ArtistAlbum album) async {
     final providerId = album.providerId;
     if (providerId != null && providerId.isNotEmpty) {
-      final resourceId = _stripPrefixedResourceId(album.id);
+      final resourceId = stripPrefixedResourceId(album.id);
       final metadata = await PlatformBridge.getProviderMetadata(
         providerId,
         'album',

lib/screens/playlist_screen.dart

@@ -10,6 +10,7 @@ import 'package:spotiflac_android/providers/library_collections_provider.dart';
 import 'package:spotiflac_android/utils/image_cache_utils.dart';
 import 'package:spotiflac_android/utils/string_utils.dart';
 import 'package:spotiflac_android/utils/nav_bar_inset.dart';
+import 'package:spotiflac_android/utils/provider_resource_ids.dart';
 import 'package:spotiflac_android/providers/settings_provider.dart';
 import 'package:spotiflac_android/providers/local_library_provider.dart';
 import 'package:spotiflac_android/widgets/download_service_picker.dart';
@@ -57,23 +58,10 @@ class _PlaylistScreenState extends ConsumerState<PlaylistScreen> {
   String? get _headerVideoUrl =>
       _resolvedHeaderVideoUrl ?? widget.headerVideoUrl;
 
-  String? _legacyProviderIdFromResourceId(String value) {
-    if (value.startsWith('deezer:')) return 'deezer';
-    if (value.startsWith('qobuz:')) return 'qobuz';
-    if (value.startsWith('tidal:')) return 'tidal';
-    return null;
-  }
 
-  String _stripPrefixedResourceId(String value) {
-    final colonIndex = value.indexOf(':');
-    if (colonIndex <= 0 || colonIndex == value.length - 1) {
-      return value;
-    }
-    return value.substring(colonIndex + 1);
-  }
 
   String? _metadataProviderId(String playlistId) {
-    final providerId = _legacyProviderIdFromResourceId(playlistId);
+    final providerId = legacyProviderIdFromResourceId(playlistId);
     if (providerId == null) return null;
     final effective = resolveEffectiveMetadataProvider(
       providerId,
@@ -83,7 +71,7 @@ class _PlaylistScreenState extends ConsumerState<PlaylistScreen> {
   }
 
   String _metadataResourceId(String providerId, String playlistId) {
-    return _stripPrefixedResourceId(playlistId);
+    return stripPrefixedResourceId(playlistId);
   }
 
   String? _recommendedDownloadService() {
@@ -109,7 +97,7 @@ class _PlaylistScreenState extends ConsumerState<PlaylistScreen> {
     }
 
     final trackId = _tracks.firstOrNull?.id ?? '';
-    final trackProviderId = _legacyProviderIdFromResourceId(trackId);
+    final trackProviderId = legacyProviderIdFromResourceId(trackId);
     if (trackProviderId != null) {
       return resolveEffectiveDownloadService(
         trackProviderId,

lib/utils/provider_resource_ids.dart

@@ -0,0 +1,18 @@
+/// Maps a legacy prefixed resource id (e.g. "deezer:123") to its provider id,
+/// or null when the value carries no known provider prefix.
+String? legacyProviderIdFromResourceId(String value) {
+  if (value.startsWith('deezer:')) return 'deezer';
+  if (value.startsWith('qobuz:')) return 'qobuz';
+  if (value.startsWith('tidal:')) return 'tidal';
+  if (value.startsWith('spotify:')) return 'spotify';
+  return null;
+}
+
+/// Strips a leading "provider:" prefix from a resource id, if present.
+String stripPrefixedResourceId(String value) {
+  final colonIndex = value.indexOf(':');
+  if (colonIndex <= 0 || colonIndex == value.length - 1) {
+    return value;
+  }
+  return value.substring(colonIndex + 1);
+}