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/Program.cs

@@ -19,6 +19,7 @@ builder.Services.AddScoped<ZugferdExtractorService>();
 builder.Services.AddScoped<ZugferdParserService>();
 builder.Services.AddScoped<ZugferdImportService>();
 builder.Services.AddScoped<AttachmentViewerService>();
+builder.Services.AddScoped<PdfSigningService>();
 
 var app = builder.Build();