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/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);
+}