Replace SignatureDto with new Signature record

Replaced the `SignatureDto` record with a new `Signature` record to provide a more robust representation of signature data. Updated `MappingProfile` to map `Signature` to `DocReceiverElement` and removed the old `SignatureDto` mapping. Updated `SigningCommand` to use `IEnumerable<Signature>` instead of `IEnumerable<SignatureDto>`. Removed the old `MappingProfile` class and adjusted namespaces and `using` directives accordingly. These changes improve maintainability and streamline signature handling.

EnvelopeGenerator.Application/Common/Dto/Signature.cs

@@ -0,0 +1,65 @@
+namespace EnvelopeGenerator.Application.Common.Dto;
+
+/// <summary>
+/// Represents a captured signature with metadata created by the receiver in the signature popup.
+/// This model holds the signature image (as base64 data URL) along with signer information
+/// used for rendering applied signatures on the PDF canvas.
+/// </summary>
+/// <remarks>
+/// <b>Used in:</b> EnvelopeViewer.razor signature popup workflow
+/// <br/>
+/// <b>Creation:</b> User draws/types/uploads signature and fills required fields
+/// </remarks>
+public sealed record Signature
+{
+    /// <summary>
+    /// TBDD_DOCUMENT_RECEIVER_ELEMENT.ID - identifies the specific signature field on the PDF page.
+    /// </summary>
+    public required int ElementId { get; init; }
+
+    /// <summary>
+    /// Base64-encoded data URL of the signature image.
+    /// <br/>
+    /// <b>Format:</b> <c>data:image/png;base64,iVBORw0KG...</c>
+    /// <br/>
+    /// <b>Source:</b> Canvas.toDataURL() from signature pad (draw/text/image tabs)
+    /// <br/>
+    /// <b>Usage:</b> Set as <c>img.src</c> in applied signature overlay
+    /// </summary>
+    public required string DataUrl { get; init; }
+
+    /// <summary>
+    /// Full name of the signer (first and last name).
+    /// <br/>
+    /// <b>Required:</b> Yes (validated in popup)
+    /// <br/>
+    /// <b>Example:</b> "Max Mustermann"
+    /// </summary>
+    public required string FullName { get; init; }
+
+    private readonly string? _position = null;
+
+    /// <summary>
+    /// Job title or position of the signer.
+    /// <br/>
+    /// <b>Required:</b> No (optional field)
+    /// <br/>
+    /// <b>Example:</b> "Geschäftsführer" or empty string
+    /// </summary>
+    public string? Position
+    {
+        get => _position;
+        init => _position = string.IsNullOrWhiteSpace(value) ? value : null;
+    }
+
+    /// <summary>
+    /// Location/place where the signature was created.
+    /// <br/>
+    /// <b>Required:</b> Yes (validated in popup)
+    /// <br/>
+    /// <b>Display:</b> Shown with current date in German format (dd.MM.yyyy)
+    /// <br/>
+    /// <b>Example:</b> "Berlin" ? rendered as "Berlin, 26.01.2025"
+    /// </summary>
+    public required string Place { get; init; }
+}