Fix up usings for the project files.

wolverine/a-frame-architecture/FinancialApi.Domain/Account.cs

@@ -1,4 +1,6 @@
-namespace FinancialApi.Domain;
+using System;
+
+namespace FinancialApi.Domain;
 
 public record Account(int Id, decimal Balance, string AccountType)
 {

wolverine/a-frame-architecture/FinancialApi.Domain/BalancedJournal.cs

@@ -1,3 +1,7 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
 namespace FinancialApi.Domain;
 
 public record BalancedJournal

wolverine/a-frame-architecture/FinancialApi.Domain/JournalEntry.cs

@@ -1,3 +1,6 @@
+using System;
+using System.Collections.Generic;
+
 namespace FinancialApi.Domain;
 
 public record JournalEntry