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');
+      }
+    }
+  }
+}

lib/core/models/config.dart

@@ -0,0 +1,37 @@
+/// Application configuration model.
+///
+/// Stores user preferences that are persisted across sessions,
+/// such as display mode and fullscreen settings.
+class Config {
+  /// Storage keys for persistence
+  static const String keyTwoPageMode = 'twoPageMode';
+  static const String keyFullscreen = 'fullscreen';
+
+  /// Whether to display two pages side-by-side (for tablets/landscape).
+  bool twoPageMode;
+
+  /// Whether the app is in fullscreen mode.
+  bool fullscreen;
+
+  Config({
+    required this.twoPageMode,
+    required this.fullscreen,
+  });
+
+  /// Creates a default configuration with all options disabled.
+  factory Config.defaultConfig() => Config(
+        twoPageMode: false,
+        fullscreen: false,
+      );
+
+  /// Creates a copy of this config with optional overrides.
+  Config copyWith({
+    bool? twoPageMode,
+    bool? fullscreen,
+  }) {
+    return Config(
+      twoPageMode: twoPageMode ?? this.twoPageMode,
+      fullscreen: fullscreen ?? this.fullscreen,
+    );
+  }
+}

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(),
+      };
+}