optimized and add diagnostic feature

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; }
+    }
+}