Complete refactor to clean up project

lib/core/models/sheet.dart

@@ -0,0 +1,40 @@
+/// Data model representing a sheet music document.
+///
+/// A [Sheet] contains metadata about a piece of sheet music,
+/// including its title, composer information, and timestamps.
+class Sheet {
+  final String uuid;
+  String name;
+  String composerUuid;
+  String composerName;
+  DateTime updatedAt;
+
+  Sheet({
+    required this.uuid,
+    required this.name,
+    required this.composerUuid,
+    required this.composerName,
+    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(),
+      name: json['title'],
+      composerUuid: json['composer_uuid']?.toString() ?? '',
+      composerName: composer?['name'] ?? 'Unknown',
+      updatedAt: DateTime.parse(json['updated_at']),
+    );
+  }
+
+  /// Converts this sheet to a JSON map for API requests.
+  Map<String, dynamic> toJson() => {
+        'uuid': uuid,
+        'title': name,
+        'composer_uuid': composerUuid,
+        'composer_name': composerName,
+        'updated_at': updatedAt.toIso8601String(),
+      };
+}