optimized and add diagnostic feature

Billing.Shared/Models/Account.cs

@@ -6,6 +6,9 @@ namespace Billing.Models
     {
         private const string ICON_DEFAULT = "ic_default";
 
+        private static Account empty;
+        public static Account Empty => empty ??= new() { Id = -1 };
+
         [PrimaryKey, AutoIncrement]
         public int Id { get; set; }
         public string Icon { get; set; } = ICON_DEFAULT;

Billing.Shared/Models/Category.cs

@@ -8,6 +8,9 @@ namespace Billing.Models
         private const string ICON_DEFAULT = "ic_default";
         private const long TRANSPARENT_COLOR = 0x00ffffffL;
 
+        private static Category empty;
+        public static Category Empty => empty ??= new() { Id = -1 };
+
         [PrimaryKey, AutoIncrement]
         public int Id { get; set; }
         public CategoryType Type { get; set; }

Billing.Shared/Models/Logs.cs

@@ -0,0 +1,14 @@
+using System;
+using SQLite;
+
+namespace Billing.Models
+{
+    public class Logs : IIdItem
+    {
+        [PrimaryKey, AutoIncrement]
+        public int Id { get; set; }
+        public DateTime LogTime { get; set; }
+        public string Category { get; set; }
+        public string Detail { get; set; }
+    }
+}