Refactor PDF viewing and integrate DevExpress support

- Removed folder reference for "Controllers" in the project file.
- Updated `ViewPdf.cshtml` to include layout and clarify PDF output.
- Enhanced `ViewPdf.cshtml.cs` with improved logging and error handling.
- Added MVC controller services in `Program.cs` for DevExpress functionality.
- Introduced `DocumentViewerController` to manage document viewing requests.

DXApp.TemplateKitProject/Pages/Invoices/ViewPdf.cshtml

@@ -1,2 +1,6 @@
 @page
-@model DXApp.TemplateKitProject.Pages.Invoices.ViewPdfModel
+@model DXApp.TemplateKitProject.Pages.Invoices.ViewPdfModel
+@{
+    Layout = "_Layout";
+    // Diese Seite liefert keinen HTML-Content; der Handler gibt die PDF zurück.
+}