Add PDF signing feature with UI and backend support

Introduced a new `PdfSigningService` for digitally signing PDFs
using a PFX certificate and DevExpress's `PdfDocumentSigner`.
Updated `Program.cs` to register the service and added a new
configuration section `PdfSigning` in `appsettings.json` for
certificate management.

Added a Razor Page `TestSignature.cshtml` and its page model
`TestSignature.cshtml.cs` to provide a user interface for testing
PDF signing. The page includes file upload, signature validation,
and result display functionality.

Implemented supporting classes `PdfSigningOptions` for signature
customization and `SignatureInformation` for extracting and
displaying signature details, including signer name, location,
and certificate validity.

DXApp.TemplateKitProject/Pages/TestSignature.cshtml

@@ -0,0 +1,90 @@
+@page
+@model DXApp.TemplateKitProject.Pages.TestSignatureModel
+@{
+    ViewData["Title"] = "PDF-Signatur testen";
+}
+
+<div class="content-block">
+    <h2><i class="dx-icon-key"></i> PDF-Signatur Test</h2>
+    <p class="text-muted">Einfacher Test zum Signieren einer PDF mit dem DevExpress PdfDocumentSigner.</p>
+
+    <form method="post" enctype="multipart/form-data" class="mt-4">
+        <div class="mb-3">
+            <label for="pdfFile" class="form-label">PDF-Datei auswählen</label>
+            <input type="file" class="form-control" id="pdfFile" name="PdfFile" accept=".pdf" required />
+            <span asp-validation-for="PdfFile" class="text-danger"></span>
+        </div>
+
+        <button type="submit" class="btn btn-primary">
+            <i class="dx-icon-key"></i> PDF signieren
+        </button>
+    </form>
+
+    @if (Model.Success)
+    {
+        <div class="alert alert-success mt-4">
+            <h5><i class="dx-icon-check"></i> ? PDF erfolgreich signiert!</h5>
+            <p class="mb-2">
+                <strong>Original:</strong> @Model.OriginalFileName (@Model.OriginalSizeKb KB)<br />
+                <strong>Signiert:</strong> @Model.SignedFileName (@Model.SignedSizeKb KB)<br />
+                <strong>Gespeichert:</strong> <code>@Model.OutputPath</code>
+            </p>
+        </div>
+
+        @if (Model.SignatureInfo != null && Model.SignatureInfo.Count > 0)
+        {
+            <div class="alert alert-info mt-3">
+                <h6><i class="dx-icon-info"></i> Signatur-Details:</h6>
+                @foreach (var sig in Model.SignatureInfo)
+                {
+                    <div class="mb-3 p-2 border-start border-3 border-primary">
+                        <strong>Signatur:</strong> @sig.FieldName<br />
+                        <strong>Unterzeichner:</strong> @sig.SignerName<br />
+                        <strong>Ort:</strong> @sig.Location<br />
+                        <strong>Grund:</strong> @sig.Reason<br />
+                        <strong>Datum:</strong> @sig.Date.ToString("dd.MM.yyyy HH:mm:ss")<br />
+                        <strong>Signatur gültig:</strong> 
+                        @if (sig.IsSignatureValid)
+                        {
+                            <span class="badge bg-success">? Ja</span>
+                        }
+                        else
+                        {
+                            <span class="badge bg-danger">? Nein</span>
+                        }
+                        <br />
+                        <strong>Zertifikat gültig:</strong> 
+                        @if (sig.IsCertificateValid)
+                        {
+                            <span class="badge bg-success">? Ja</span>
+                        }
+                        else
+                        {
+                            <span class="badge bg-danger">? Nein</span>
+                        }
+                        <br />
+                        <strong>Zertifikat gültig bis:</strong> @sig.CertificateValidUntil.ToString("dd.MM.yyyy")<br />
+                        <small class="text-muted">Issuer: @sig.CertificateIssuer</small>
+                    </div>
+                }
+            </div>
+        }
+    }
+
+    @if (!string.IsNullOrEmpty(Model.ErrorMessage))
+    {
+        <div class="alert alert-danger mt-4">
+            <i class="dx-icon-warning"></i> <strong>Fehler:</strong> @Model.ErrorMessage
+        </div>
+    }
+
+    <div class="alert alert-light mt-4">
+        <h6>?? Test-Schritte:</h6>
+        <ol class="mb-0">
+            <li>PDF-Datei auswählen (z.B. eine ZUGFeRD-Rechnung aus <code>/Invoices</code>)</li>
+            <li>Auf "PDF signieren" klicken</li>
+            <li>Signierte PDF wird in <code>C:\PdfResults</code> gespeichert</li>
+            <li>Mit Adobe Acrobat Reader öffnen und Signatur prüfen</li>
+        </ol>
+    </div>
+</div>