Add signature ink handling and DevExpress integration

Introduced `ISignatureInkPayload` interface to standardize signature ink data and metadata. Updated `DocReceiverElementDto` and `DocReceiverElement` to implement this interface. Enhanced `IPdfPageImageRenderer` with methods for signature preview generation and image format validation.

Replaced `PdfSharp` with `DevExpress` for PDF rendering in `ReceiverSignedPage.razor`. Added `DevSignatureInkController` with endpoints for downloading raw and composed signature images. Updated project files to remove `SixLabors.ImageSharp` and integrate DevExpress.

Added configuration for development endpoints in `appsettings.json`. Improved exception handling, validation, and code maintainability.

EnvelopeGenerator.Domain/Interfaces/ISignatureInkPayload.cs

@@ -0,0 +1,28 @@
+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface ISignatureInkPayload
+    {
+#if nullable
+        byte[]? Ink { get; }
+
+        string? FullName { get; }
+
+        string? Position { get; }
+
+        string? Place { get; }
+#else
+        byte[] Ink { get; }
+
+        string FullName { get; }
+
+        string Position { get; }
+
+        string Place { get; }
+#endif
+        DateTime? ChangedWhen { get; }
+    }
+}