Add SignatureDtoExtensions with Convert<T> method

Introduce a new static class `SignatureDtoExtensions` that adds
a `Convert<T>` extension method for collections of `SignatureDto`
objects. This method converts all `SignatureDto` instances in a
collection to a specified `UnitOfLength`, modifying them in place
and returning the same collection.

The method includes:
- A generic constraint to support any `IEnumerable<SignatureDto>`.
- Null checks to ensure the collection is not null.
- Comprehensive XML documentation with usage examples and notes.

EnvelopeGenerator.ReceiverUI/Models/SignatureDto.cs

@@ -66,3 +66,36 @@ public class SignatureDto
         }
     }
 }
+
+public static class SignatureDtoExtensions
+{
+    /// <summary>
+    /// Converts all signatures in the collection to the specified unit of length.
+    /// </summary>
+    /// <typeparam name="T">Type of the collection (IEnumerable, List, etc.)</typeparam>
+    /// <param name="signatures">Collection of SignatureDto objects to convert.</param>
+    /// <param name="unitOfLength">Target unit of measurement (Inch or Point).</param>
+    /// <returns>The same collection with all signatures converted to the specified unit.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when signatures collection is null.</exception>
+    /// <remarks>
+    /// <b>Usage:</b>
+    /// <code>
+    /// var signatures = await SignatureService.GetAsync(envelopeKey);
+    /// var convertedSignatures = signatures.ConvertAll(UnitOfLength.Point);
+    /// </code>
+    /// <b>Note:</b> This method modifies each SignatureDto object in place and returns the same collection.
+    /// </remarks>
+    public static T Convert<T>(this T signatures, UnitOfLength unitOfLength) 
+        where T : IEnumerable<SignatureDto>
+    {
+        if (signatures == null)
+            throw new ArgumentNullException(nameof(signatures));
+
+        foreach (var signature in signatures)
+        {
+            signature.Convert(unitOfLength);
+        }
+
+        return signatures;
+    }
+}