Enhance AutoMapper mappings and add base64 decoding

Updated `MappingProfile` to map `Signature.DataUrl` to
`DocReceiverElement.Ink` using the new `MapDataUrlToRequiredBytes`
extension method. Added `MapDataUrlToRequiredBytes` to handle
base64-encoded data URLs, converting them to byte arrays.

Introduced a `using System;` directive in `AutoMapperAuditingExtensions.cs`
to support `DateTime`. Retained `MapChangedWhen` functionality while
extending mapping capabilities for handling base64 data URLs.

EnvelopeGenerator.Application/Common/Extensions/AutoMapperAuditingExtensions.cs

@@ -1,5 +1,6 @@
 using AutoMapper;
 using EnvelopeGenerator.Domain.Interfaces.Auditing;
+using System;
 
 namespace EnvelopeGenerator.Application.Common.Extensions;
 
@@ -21,4 +22,21 @@ public static class AutoMapperAuditingExtensions
     public static IMappingExpression<TSource, TDestination> MapChangedWhen<TSource, TDestination>(this IMappingExpression<TSource, TDestination> expression)
         where TDestination : IHasChangedWhen
         => expression.ForMember(dest => dest.ChangedWhen, opt => opt.MapFrom(_ => DateTime.Now));
+
+    /// <summary>
+    /// Converts a base64 data URL string to a byte array.
+    /// Handles data URLs in the format: "data:image/png;base64,iVBORw0KG..."
+    /// </summary>
+    /// <param name="dataUrl">The base64 data URL string from Canvas.toDataURL()</param>
+    /// <returns>The decoded byte array, or null if the input is null or empty</returns>
+    public static byte[]? MapDataUrlToRequiredBytes(this string dataUrl)
+    {
+        // Remove data URL prefix (e.g., "data:image/png;base64,")
+        var base64Index = dataUrl.IndexOf(',', StringComparison.Ordinal);
+        if (base64Index == -1)
+            throw new ArgumentException("Invalid data URL format. Unable to extract base64 data.", nameof(dataUrl));
+
+        var base64Data = dataUrl[(base64Index + 1)..];
+        return Convert.FromBase64String(base64Data);
+    }
 }