Add e-signature support and improve PDF signing process

Introduced `ESignParams` for configurable e-signature settings,
including `ApplicationName` and `DefaultReason`. Updated
`DocReceiverElementDto` and `PdfSignOptionsDto` to support
new properties. Registered `ESignParams` in DI container.

Refactored `SignPdfByElementCommand` to include new fields
and improved error handling. Added `SignPdfByEnvelopeReceiverCommand`
to enable signing documents for envelope receivers.

Replaced DevExpress with PdfSharp for PDF rendering in
`ReceiverSignedPage.razor`, improving layout and flexibility.
Enhanced `PdfSignatureController` with a new endpoint for
signing documents by envelope key.

Added `DataIntegrityException` for better error handling.
Updated `appsettings.json` with `ESignParams` configuration.

EnvelopeGenerator.Application/PdfSigning/SignPdfByEnvelopeReceiverCommand.cs

@@ -0,0 +1,89 @@
+using AutoMapper;
+using DigitalData.Core.Abstraction.Application.Repository;
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Enums;
+using EnvelopeGenerator.Application.Documents.Queries;
+using EnvelopeGenerator.Application.EnvelopeReceivers.Queries;
+using EnvelopeGenerator.Domain.Entities;
+using EnvelopeGenerator.Domain.Exceptions;
+using MediatR;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.PdfSigning;
+
+/// <summary>
+/// 
+/// </summary>
+public record SignPdfByEnvelopeReceiverCommand : IRequest<Stream>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public int EnvelopeId { get; init; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public int ReceiverId { get; init; }
+}
+
+/// <summary>
+/// 
+/// </summary>
+public class SignPdfByEnvelopeReceiverCommandHandler(IRepository<DocReceiverElement> Repo, IMapper Mapper, ISender Sender) : IRequestHandler<SignPdfByEnvelopeReceiverCommand, Stream>
+{
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    /// <exception cref="NotFoundException"></exception>
+    public async Task<Stream> Handle(SignPdfByEnvelopeReceiverCommand request, CancellationToken cancel)
+    {
+        cancel.ThrowIfCancellationRequested();
+
+        // read elements
+        var elements = await Repo.Where(el => el.ReceiverId == request.ReceiverId && el.Document.EnvelopeId == request.EnvelopeId)
+            .Include(e => e.Receiver)
+            .ToListAsync(cancel) ?? throw new NotFoundException($"No elements found for Envelope ID {request.EnvelopeId} and Receiver ID {request.ReceiverId}.");
+        var elementDtos = Mapper.Map<List<DocReceiverElementDto>>(elements)
+            .OrderBy(e => e.Page)
+            .ThenBy(e => e.Id)
+            .ToList();
+
+        // read secrets for phone number
+        var erSecrets = await Sender.Send(new ReadEnvelopeReceiverSecretQuery()
+        {
+            Envelope = new() { Id = request.EnvelopeId },
+            Receiver = new() { Id = request.ReceiverId }
+        }, cancel) ?? throw new DataIntegrityException($"No secrets found for Envelope ID {request.EnvelopeId} and Receiver ID {request.ReceiverId}.");
+
+        // read document
+        var unsignedDoc = await Sender.Send(new ReadDocumentQuery(EnvelopeId: request.EnvelopeId), cancel);
+        var unsignedDocBytes = unsignedDoc.ByteData ?? throw new NotFoundException($"Document not found for Envelope ID {request.EnvelopeId} and Receiver ID {request.ReceiverId}.");
+                
+        Stream doc = new MemoryStream(unsignedDocBytes);
+
+        // sign document for each element
+        foreach (var element in elementDtos)
+        {
+            var lastElement = element == elementDtos.Last();
+
+            var signedDoc = await Sender.Send(new SignPdfByElementCommand
+            {
+                Document = doc,
+                Element = element,
+                CertificationLevel = lastElement ? PdfSignatureCertificationLevel.NoChangesAllowed : PdfSignatureCertificationLevel.FillFormsAndAnnotate,
+                Phone = erSecrets.PhoneNumber
+            }, cancel);
+
+            await doc.DisposeAsync();
+            doc = signedDoc;
+        }
+
+        return doc;
+    }
+}