feat(core): extend balance model to store currency code

src/FiscalOS.Core/Accounts/Balance.cs

@@ -5,17 +5,23 @@ public sealed class Balance : Entity
   public Guid AccountId { get; init; }
   public decimal Current { get; init; }
   public decimal Available { get; init; }
+  public string CurrencyCode { get; init; } = string.Empty;
 
   private Balance()
   {
   }
 
-  public static Balance From(decimal current, decimal available)
+  public static Balance From(
+    decimal current,
+    decimal available,
+    string currencyCode
+  )
   {
     return new()
     {
       Current = current,
       Available = available,
+      CurrencyCode = currencyCode,
     };
   }
 }