Implement offline mode

lib/core/models/change.dart

@@ -13,23 +13,35 @@ enum ChangeType {
 /// Represents a single pending change to be synced with the server.
 ///
 /// Changes are stored locally when offline and applied once
-/// the device regains connectivity.
+/// the device regains connectivity. Each change has a [createdAt] timestamp
+/// that the server uses to resolve conflicts between devices.
 class Change {
   final ChangeType type;
   final String sheetUuid;
   final String value;
+  final DateTime createdAt;
 
   Change({
     required this.type,
     required this.sheetUuid,
     required this.value,
-  });
+    DateTime? createdAt,
+  }) : createdAt = createdAt ?? DateTime.now();
 
   /// Serializes this change to a map for storage.
   Map<String, dynamic> toMap() => {
         'type': type.index,
         'sheetUuid': sheetUuid,
         'value': value,
+        'createdAt': createdAt.toIso8601String(),
+      };
+
+  /// Serializes this change to JSON for API requests.
+  Map<String, dynamic> toJson() => {
+        'type': type.name,
+        'sheetUuid': sheetUuid,
+        'value': value,
+        'createdAt': createdAt.toIso8601String(),
       };
 
   /// Deserializes a change from a stored map.
@@ -41,6 +53,9 @@ class Change {
       type: ChangeType.values[map['type']],
       sheetUuid: map['sheetUuid'],
       value: map['value'],
+      createdAt: map['createdAt'] != null
+          ? DateTime.parse(map['createdAt'])
+          : DateTime.now(),
     );
   }
 }