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/Details.cshtml

@@ -75,9 +75,9 @@ else
                     _ => "📎"
                 };
 
-                <a href="/Invoices/ViewAttachment?filePath=@Uri.EscapeDataString(attachment.SavedFilePath)" 
+                <a href="javascript:void(0);" 
                    class="list-group-item list-group-item-action d-flex justify-content-between align-items-center"
-                   target="_blank">
+                   onclick="openAttachmentViewer('@attachment.OriginalFileName', '@Uri.EscapeDataString(attachment.SavedFilePath)', '@extension')">
                     <div>
                         <span class="me-2">@icon</span>
                         <strong>@attachment.OriginalFileName</strong>
@@ -115,6 +115,20 @@ else
         return '<iframe id=""pdf-iframe"" style=""width:100%;height:100%;border:none;""></iframe>';
     }"))
 )
+
+@(Html.DevExtreme().Popup()
+    .ID("attachment-viewer-popup")
+    .Title("Anhang")
+    .Width("90%")
+    .Height("90%")
+    .ShowCloseButton(true)
+    .OnHiding("onAttachmentPopupHiding")
+    .Shading(true)
+    .ShadingColor("rgba(0, 0, 0, 0.5)")
+    .ContentTemplate(new JS(@"function() {
+        return '<div id=""attachment-content"" style=""width:100%;height:100%;overflow:auto;""></div>';
+    }"))
+)
 }
 
 <style>
@@ -160,4 +174,104 @@ else
         // iframe src leeren beim Schließen → verhindert dass PDF im Hintergrund weiter läuft
         $('#pdf-iframe').attr('src', '');
     }
+
+    function openAttachmentViewer(fileName, encodedFilePath, extension) {
+        var filePath = decodeURIComponent(encodedFilePath);
+        var popup = $('#attachment-viewer-popup').dxPopup('instance');
+        var $content = $('#attachment-content');
+
+        // Popup-Titel setzen
+        popup.option('title', fileName);
+
+        // Z-Index setzen
+        popup.option('container', undefined);
+        popup.option('position', { my: 'center', at: 'center', of: window });
+
+        // Content basierend auf Dateityp laden
+        popup.option('onShown', function () {
+            $content.html('<div class="text-center p-5"><div class="spinner-border text-primary" role="status"><span class="visually-hidden">Laden...</span></div></div>');
+
+            // Z-Index sicherstellen
+            $('.dx-popup-wrapper').css('z-index', '10500');
+            $('.dx-overlay-shader').css('z-index', '10499');
+
+            if (extension === '.pdf') {
+                // PDF mit PDF.js anzeigen
+                var pdfUrl = window.location.origin + '/Invoices/ViewAttachment?handler=Download&filePath=' + encodedFilePath;
+                var viewerUrl = '/js/pdfjs/web/viewer.html?file=' + encodeURIComponent(pdfUrl);
+                $content.html('<iframe style="width:100%;height:100%;border:none;" src="' + viewerUrl + '"></iframe>');
+            } 
+            else if (extension === '.xml' || extension === '.txt') {
+                // Text/XML laden und mit Syntax-Highlighting anzeigen
+                $.get('/Invoices/ViewAttachment?filePath=' + encodedFilePath, function(data) {
+                    if (extension === '.xml') {
+                        // CodeMirror für XML
+                        var textarea = $('<textarea id="code-viewer"></textarea>');
+                        $content.html('').append(textarea);
+                        
+                        // CodeMirror laden (falls noch nicht geladen)
+                        if (typeof CodeMirror === 'undefined') {
+                            $('<link>')
+                                .attr('rel', 'stylesheet')
+                                .attr('href', 'https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.65.16/codemirror.min.css')
+                                .appendTo('head');
+                            $('<link>')
+                                .attr('rel', 'stylesheet')
+                                .attr('href', 'https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.65.16/theme/monokai.min.css')
+                                .appendTo('head');
+                            
+                            $.getScript('https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.65.16/codemirror.min.js', function() {
+                                $.getScript('https://cdnjs.cloudflare.com/ajax/libs/codemirror/5.65.16/mode/xml/xml.min.js', function() {
+                                    initCodeMirror(data);
+                                });
+                            });
+                        } else {
+                            initCodeMirror(data);
+                        }
+
+                        function initCodeMirror(content) {
+                            CodeMirror(document.getElementById('attachment-content'), {
+                                value: content,
+                                mode: 'xml',
+                                lineNumbers: true,
+                                readOnly: true,
+                                theme: 'monokai',
+                                lineWrapping: true
+                            });
+                        }
+                    } else {
+                        // Plain Text
+                        $content.html('<pre style="background-color: #f5f5f5; padding: 15px; border-radius: 5px; height: 100%; overflow: auto; margin: 0;">' + 
+                            $('<div>').text(data).html() + '</pre>');
+                    }
+                }).fail(function() {
+                    $content.html('<div class="alert alert-danger m-3">Fehler beim Laden der Datei.</div>');
+                });
+            }
+            else if (extension === '.jpg' || extension === '.jpeg' || extension === '.png' || extension === '.gif') {
+                // Bild anzeigen
+                var imageUrl = window.location.origin + '/Invoices/ViewAttachment?handler=Download&filePath=' + encodedFilePath;
+                $content.html('<div class="text-center p-3" style="height: 100%; display: flex; align-items: center; justify-content: center;">' +
+                    '<img src="' + imageUrl + '" alt="' + fileName + '" class="img-fluid" style="max-height: 100%; max-width: 100%; object-fit: contain;">' +
+                    '</div>');
+            }
+            else {
+                // Nicht unterstützter Typ → Download anbieten
+                var downloadUrl = window.location.origin + '/Invoices/ViewAttachment?handler=Download&filePath=' + encodedFilePath;
+                $content.html('<div class="alert alert-info m-3">' +
+                    '<h5>Dieser Dateityp kann nicht angezeigt werden.</h5>' +
+                    '<p>Bitte laden Sie die Datei herunter:</p>' +
+                    '<a href="' + downloadUrl + '" class="btn btn-primary" download="' + fileName + '">' +
+                    '<i class="dx-icon-download"></i> Datei herunterladen' +
+                    '</a></div>');
+            }
+        });
+
+        popup.show();
+    }
+
+    function onAttachmentPopupHiding() {
+        // Content leeren
+        $('#attachment-content').html('');
+    }
 </script>

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();