Sheet model: adapt to reduced data model of server

lib/core/models/sheet.dart

@@ -5,26 +5,22 @@
 class Sheet {
   final String uuid;
   String name;
-  String composerUuid;
-  String composerName;
+  String composer;
   DateTime updatedAt;
 
   Sheet({
     required this.uuid,
     required this.name,
-    required this.composerUuid,
-    required this.composerName,
+    required this.composer,
     required this.updatedAt,
   });
 
   /// Creates a [Sheet] from a JSON map returned by the API.
   factory Sheet.fromJson(Map<String, dynamic> json) {
-    final composer = json['composer'] as Map<String, dynamic>?;
     return Sheet(
-      uuid: json['uuid'].toString(),
+      uuid: json['uuid'],
       name: json['title'],
-      composerUuid: json['composer_uuid']?.toString() ?? '',
-      composerName: composer?['name'] ?? 'Unknown',
+      composer: json['composer'],
       updatedAt: DateTime.parse(json['updated_at']),
     );
   }
@@ -33,8 +29,7 @@ class Sheet {
   Map<String, dynamic> toJson() => {
     'uuid': uuid,
     'title': name,
-        'composer_uuid': composerUuid,
-        'composer_name': composerName,
+    'composer': composer,
     'updated_at': updatedAt.toIso8601String(),
   };
 }