Complete refactor to clean up project

lib/core/models/change.dart

@@ -0,0 +1,96 @@
+import 'dart:collection';
+
+import 'sheet.dart';
+
+/// Types of changes that can be queued for server synchronization.
+enum ChangeType {
+  sheetNameChange,
+  composerNameChange,
+  addTagChange,
+  removeTagChange,
+}
+
+/// Represents a single pending change to be synced with the server.
+///
+/// Changes are stored locally when offline and applied once
+/// the device regains connectivity.
+class Change {
+  final ChangeType type;
+  final String sheetUuid;
+  final String value;
+
+  Change({
+    required this.type,
+    required this.sheetUuid,
+    required this.value,
+  });
+
+  /// Serializes this change to a map for storage.
+  Map<String, dynamic> toMap() => {
+        'type': type.index,
+        'sheetUuid': sheetUuid,
+        'value': value,
+      };
+
+  /// Deserializes a change from a stored map.
+  ///
+  /// Note: Adding new [ChangeType] values may cause issues with
+  /// previously stored changes that use index-based serialization.
+  factory Change.fromMap(Map<dynamic, dynamic> map) {
+    return Change(
+      type: ChangeType.values[map['type']],
+      sheetUuid: map['sheetUuid'],
+      value: map['value'],
+    );
+  }
+}
+
+/// A queue of pending changes to be synchronized with the server.
+///
+/// Changes are stored in FIFO order (oldest first) and applied
+/// to sheets in sequence when syncing.
+class ChangeQueue {
+  final Queue<Change> _queue = Queue();
+
+  ChangeQueue();
+
+  /// Adds a change to the end of the queue.
+  void addChange(Change change) {
+    _queue.addLast(change);
+  }
+
+  /// Returns the number of pending changes.
+  int get length => _queue.length;
+
+  /// Whether the queue has any pending changes.
+  bool get isEmpty => _queue.isEmpty;
+
+  /// Whether the queue has pending changes.
+  bool get isNotEmpty => _queue.isNotEmpty;
+
+  /// Applies all queued changes to the provided list of sheets.
+  ///
+  /// Each change modifies the corresponding sheet's properties
+  /// based on the change type.
+  void applyToSheets(List<Sheet> sheets) {
+    for (final change in _queue) {
+      final sheet = sheets.firstWhere(
+        (s) => s.uuid == change.sheetUuid,
+        orElse: () => throw StateError(
+          'Sheet with UUID ${change.sheetUuid} not found',
+        ),
+      );
+
+      switch (change.type) {
+        case ChangeType.sheetNameChange:
+          sheet.name = change.value;
+        case ChangeType.composerNameChange:
+          sheet.composerName = change.value;
+        case ChangeType.addTagChange:
+          throw UnimplementedError('Tag support not yet implemented');
+        case ChangeType.removeTagChange:
+          throw UnimplementedError('Tag support not yet implemented');
+      }
+    }
+  }
+}