Add the Income and Asset classes.

lib/models/asset.dart

@@ -0,0 +1,19 @@
+import '/models/tracked_type.dart';
+
+class Asset extends TrackedType {
+  static String amountText = "Amount";
+
+  Asset({
+    super.id,
+    required super.name,
+    required super.amount,
+    required super.description,
+  });
+
+  factory Asset.fromMap(Map<String, dynamic> json) => Asset(
+        id: json['id'],
+        name: json['name'],
+        amount: json['amount'],
+        description: json['description'],
+      );
+}