Add DevExpress Document Viewer page and dependencies

Updated `_ViewImports.cshtml` to include DevExpress and DevExtreme tag helpers. Created `DocumentViewerDevExpress.cshtml` for displaying a DevExpress Document Viewer with necessary CDN links. Implemented the corresponding code-behind in `DocumentViewerDevExpress.cshtml.cs` to handle model binding and report key generation.

DXApp.TemplateKitProject/Pages/Invoices/DocumentViewerDevExpress.cshtml

@@ -0,0 +1,32 @@
+@page
+@model DXApp.TemplateKitProject.Pages.Invoices.DocumentViewerDevExpressModel
+@{
+    ViewData["Title"] = "DevExpress Document Viewer";
+}
+
+@* Dev: Für schnelle Tests verwenden wir die CDN-Assets.
+   In Production bitte lokal hosten und Lizenz einbinden. *@
+<link rel="stylesheet" href="https://cdn3.devexpress.com/jslib/25.2.7/css/dx.common.css" />
+<link rel="stylesheet" href="https://cdn3.devexpress.com/jslib/25.2.7/css/dx.light.css" />
+<script src="https://cdn3.devexpress.com/jslib/25.2.7/js/dx.all.js"></script>
+<script src="https://cdn3.devexpress.com/jslib/25.2.7/js/dx.aspnet.data.js"></script>
+
+<div class="container-fluid" style="height:100vh; padding:0;">
+    <div class="d-flex align-items-center justify-content-between p-3 border-bottom">
+        <h5 class="m-0">DevExpress Viewer — Rechnung @Model.Id</h5>
+        <div>
+            <a class="btn btn-secondary me-2" href="/Invoices">Zurück zur Übersicht</a>
+            <a class="btn btn-outline-secondary" href="/Invoices/DocumentViewer?id=@Model.Id">Baseline Viewer</a>
+        </div>
+    </div>
+
+    <div style="height:calc(100vh - 64px);">
+        @* KORRIGIERTE SYNTAX: Der WebDocumentViewer-Helper wird direkt aufgerufen. *@
+        @(Html.DevExpress().WebDocumentViewer()
+                .Name("WebDocumentViewer")
+                .Height("100%")
+                .Width("100%")
+                .Bind(Model.ReportKey)
+                )
+    </div>
+</div>

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

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class DocumentViewerDevExpressModel : PageModel
+{
+    [BindProperty(SupportsGet = true)]
+    public int Id { get; set; }
+
+    public string ReportKey => $"invoice-{Id}";
+
+    public void OnGet()
+    {
+    }
+}

DXApp.TemplateKitProject/Pages/_ViewImports.cshtml

@@ -1,4 +1,10 @@
 @using DXApp.TemplateKitProject
+@using DXApp.TemplateKitProject.Pages
 @namespace DXApp.TemplateKitProject.Pages
 @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+
+@* --- DevExpress & DevExtreme Integration --- *@
+@using DevExpress.AspNetCore
+@using DevExpress.AspNetCore.Reporting
 @using DevExtreme.AspNet.Mvc
+@addTagHelper *, DevExtreme.AspNet.Core