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.
+}

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

@@ -1,24 +1,40 @@
-using DXApp.TemplateKitProject.Data;
-using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
+using DXApp.TemplateKitProject.Data;
 using Microsoft.EntityFrameworkCore;
 
 namespace DXApp.TemplateKitProject.Pages.Invoices;
 
-public class ViewPdfModel(AppDbContext db) : PageModel
+public class ViewPdfModel : PageModel
 {
+    private readonly AppDbContext _db;
+    private readonly ILogger<ViewPdfModel> _logger;
+
+    public ViewPdfModel(AppDbContext db, ILogger<ViewPdfModel> logger)
+    {
+        _db = db;
+        _logger = logger;
+    }
+
     public async Task<IActionResult> OnGetAsync(int id)
     {
-        var invoice = await db.ZugferdInvoices
+        // Sicherheit: defensive checks, gleiche Logik wie CustomReportStorageWebExtension
+        var invoice = await _db.ZugferdInvoices
+            .AsNoTracking()
             .FirstOrDefaultAsync(i => i.Id == id);
 
-        if (invoice is null || string.IsNullOrEmpty(invoice.ResultFilePath))
+        if (invoice is null)
+            return NotFound();
+
+        if (string.IsNullOrEmpty(invoice.ResultFilePath))
             return NotFound();
 
         if (!System.IO.File.Exists(invoice.ResultFilePath))
             return NotFound();
 
         var bytes = await System.IO.File.ReadAllBytesAsync(invoice.ResultFilePath);
-        return File(bytes, "application/pdf");
+        _logger.LogInformation("ViewPdf: Invoice {Id} ausgeliefert ({Size} Bytes).", id, bytes.Length);
+
+        return File(bytes, "application/pdf", $"{Path.GetFileName(invoice.ResultFilePath)}");
     }
 }