Add helper methods for color conversion and string checksum generation

- Implemented ToColor extension method to convert predefined ColorType enums to HTML hex colors.
- Added GetChecksum extension method to generate a SHA-256 checksum from a string.
- Included conditional directive for .NET Framework compatibility.

EnvelopeGenerator.Domain/Entities/Receiver.cs

@@ -1,7 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 using System.Drawing;
-using DigitalData.Modules.Base;
 #if NETFRAMEWORK
 using System;
 using System.Collections.Generic;
@@ -80,11 +79,11 @@ namespace EnvelopeGenerator.Domain.Entities
             SignedDate == DateTime.MinValue ? "-" : SignedDate.ToString("G");
 
         [NotMapped]
-        public Color Color => Helpers.ColorTypeToColor(ColorType);
+        public Color Color => ColorType.ToColor();
 
         public string GetSignature()
         {
-            return StringEx.GetChecksum(EmailAddress?.ToUpperInvariant() ?? string.Empty);
+            return EmailAddress.ToUpperInvariant().GetChecksum();
         }
     }
 }