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.Application/Common/Dto/DocReceiverElementDto.cs

@@ -6,7 +6,7 @@ namespace EnvelopeGenerator.Application.Common.Dto;
 /// <summary>
 /// Data Transfer Object representing a positioned element assigned to a document receiver.
 /// </summary>
-public class DocReceiverElementDto : IDocReceiverElement
+public class DocReceiverElementDto : IDocReceiverElement, ISignatureInkPayload
 {
     /// <summary>
     /// Gets or sets the unique identifier of the element.
@@ -122,4 +122,41 @@ public class DocReceiverElementDto : IDocReceiverElement
     /// 
     /// </summary>
     public byte[]? Ink { get; set; }
-}
+
+    /// <summary>
+    /// Returns the stored signature bytes as an image payload with detected MIME type.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">Thrown when signature image bytes do not exist.</exception>
+    public (byte[] Bytes, string ContentType, string FileExtension) ToImage()
+    {
+        if (Ink is not { Length: > 0 })
+            throw new InvalidOperationException("Signature image (Ink) does not exist.");
+
+        if (HasPrefix(Ink, 0x89, 0x50, 0x4E, 0x47))
+            return (Ink, "image/png", ".png");
+
+        if (HasPrefix(Ink, 0xFF, 0xD8, 0xFF))
+            return (Ink, "image/jpeg", ".jpg");
+
+        if (HasPrefix(Ink, 0x52, 0x49, 0x46, 0x46) &&
+            Ink.Length >= 12 &&
+            Ink[8] == 0x57 && Ink[9] == 0x45 && Ink[10] == 0x42 && Ink[11] == 0x50)
+            return (Ink, "image/webp", ".webp");
+
+        return (Ink, "application/octet-stream", ".bin");
+    }
+
+    private static bool HasPrefix(byte[] data, params byte[] prefix)
+    {
+        if (data.Length < prefix.Length)
+            return false;
+
+        for (var i = 0; i < prefix.Length; i++)
+        {
+            if (data[i] != prefix[i])
+                return false;
+        }
+
+        return true;
+    }
+}

EnvelopeGenerator.Application/Common/Interfaces/Doc/IPdfPageImageRenderer.cs

@@ -1,8 +1,54 @@
+using DigitalData.Core.Exceptions;
 using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Domain.Interfaces;
 
 namespace EnvelopeGenerator.Application.Common.Interfaces.Doc;
 
+/// <summary>
+/// 
+/// </summary>
 public interface IPdfPageImageRenderer
 {
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="pdfBytes"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
     Task<IReadOnlyList<PdfPageImageDto>> RenderPagesAsImagesAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="element"></param>
+    /// <returns></returns>
+    byte[] ComposeSignaturePreviewPng(ISignatureInkPayload element);
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="bytes"></param>
+    /// <returns></returns>
+    public bool IsSupportedImageFormat(byte[] bytes);
 }
+
+/// <summary>
+/// 
+/// </summary>
+public static class PdfPageImageRendererExtensions
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="renderer"></param>
+    /// <param name="inkBytes"></param>
+    /// <returns></returns>
+    /// <exception cref="BadRequestException"></exception>
+    public static byte[] EnsureSupportedImageBytes(this IPdfPageImageRenderer renderer, byte[] inkBytes)
+    {
+        if (!renderer.IsSupportedImageFormat(inkBytes))
+            throw new BadRequestException("Only PNG and JPEG signature formats are supported.");
+
+        return inkBytes;
+    }
+}