Enhance PDF viewing experience with new document viewer

- Replaced button in `Details.cshtml` with a link to a new document viewer page.
- Created `DocumentViewer.cshtml` to display PDFs using an iframe, with a back button and a link to an experimental DevExpress viewer.
- Introduced `DocumentViewerModel` in `DocumentViewer.cshtml.cs` for handling invoice ID and PDF URL.
- Updated `OnGetAsync` in `ViewPdf.cshtml.cs` to set `Content-Disposition` to inline for better PDF rendering in the browser.

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

@@ -18,7 +18,6 @@ public class ViewPdfModel : PageModel
 
     public async Task<IActionResult> OnGetAsync(int id)
     {
-        // Sicherheit: defensive checks, gleiche Logik wie CustomReportStorageWebExtension
         var invoice = await _db.ZugferdInvoices
             .AsNoTracking()
             .FirstOrDefaultAsync(i => i.Id == id);
@@ -35,6 +34,10 @@ public class ViewPdfModel : PageModel
         var bytes = await System.IO.File.ReadAllBytesAsync(invoice.ResultFilePath);
         _logger.LogInformation("ViewPdf: Invoice {Id} ausgeliefert ({Size} Bytes).", id, bytes.Length);
 
-        return File(bytes, "application/pdf", $"{Path.GetFileName(invoice.ResultFilePath)}");
+        // Wichtig: keine "attachment" Content-Disposition setzen
+        // wir setzen inline (oder lassen es weg) damit Browser im Viewer darstellt
+        Response.Headers["Content-Disposition"] = $"inline; filename=\"{Path.GetFileName(invoice.ResultFilePath)}\"";
+
+        return File(bytes, "application/pdf");
     }
 }