Enhance file attachment viewing experience

Replaced direct file links with a JavaScript-based viewer
(`openAttachmentViewer`) and added a DevExtreme Popup
(`attachment-viewer-popup`) for displaying attachments in a
modal dialog. The viewer supports PDFs (via PDF.js), XML
(with CodeMirror syntax highlighting), plain text, images,
and provides a fallback for unsupported file types.

Dynamically load CodeMirror for XML files and handle errors
gracefully when loading file content. Added `onAttachmentPopupHiding`
to clear popup content on close.

Updated `ViewAttachmentModel` to return text/XML content
directly for AJAX requests, improving frontend performance
and enabling dynamic content loading.

DXApp.TemplateKitProject/Pages/Invoices/ViewAttachment.cshtml.cs

@@ -19,10 +19,17 @@ public class ViewAttachmentModel(AttachmentViewerService viewerService) : PageMo
         FileName = Path.GetFileName(filePath);
         ViewerType = viewerService.DetermineViewerType(FileName);
 
-        // Für Text/XML: Inhalt laden
+        // Für Text/XML: Inhalt direkt als Content zurückgeben (für AJAX)
         if (ViewerType == AttachmentViewerType.Xml || ViewerType == AttachmentViewerType.Text)
         {
             TextContent = System.IO.File.ReadAllText(filePath, Encoding.UTF8);
+            
+            // Wenn Request von AJAX kommt (Accept: */* oder text/plain)
+            if (Request.Headers.Accept.ToString().Contains("*/*") || 
+                Request.Headers["X-Requested-With"] == "XMLHttpRequest")
+            {
+                return Content(TextContent, "text/plain", Encoding.UTF8);
+            }
         }
 
         return Page();