Refactor Index.razor layout and add redirection logic

Simplified the `@page` directive by removing redundant entries.
Replaced the introductory section with a centered card layout
featuring a document icon, heading, description, and a
`DxLoadingPanel` with a spinner for redirection feedback.

Removed the "Helpful Resources" section with external links.
Added a new `@code` block to handle redirection logic using
`OnAfterRenderAsync`, which redirects to `/reportviewer` after
a 1200ms delay.

EnvelopeGenerator.ReceiverUI/Pages/Index.razor

@@ -1,18 +1,40 @@
-@page "/"
+@page "/"
+@inject NavigationManager NavigationManager
 
-<h2 class="pb-2">Blazor Components</h2>
-<p class="pb-2 mw-1100">
-    Our Blazor UI components will help you create intuitive and highly-refined user experiences for both Blazor Server (ASP.NET Core) and Blazor WebAssembly hosting models.
-</p>
-<div class="pb-2 mw-1100">
-    <img class="fit-width" src="images/banner.png" />
+<div class="d-flex justify-content-center align-items-center" style="min-height: 70vh;">
+    <div class="card shadow-sm border-0" style="max-width: 560px; width: 100%;">
+        <div class="card-body p-5 text-center">
+            <div class="rounded-circle bg-primary bg-opacity-10 d-inline-flex align-items-center justify-content-center mb-4" style="width: 72px; height: 72px;">
+                <span class="oi oi-document text-primary" aria-hidden="true" style="font-size: 2rem;"></span>
+            </div>
+            <h2 class="mb-3">Empfänger-UI</h2>
+            <p class="text-muted mb-4">
+                Sie werden zur Empfänger-UI weitergeleitet. Bitte warten Sie einen Moment.
+            </p>
+
+            <DxLoadingPanel Visible="true"
+                            Text=""
+                            IndicatorVisible="false"
+                            IndicatorAreaVisible="false"
+                            IsContentBlocked="false"
+                            ApplyBackgroundShading="false">
+                <div class="d-flex flex-column align-items-center gap-3 py-3">
+                    <div class="spinner-border text-primary" role="status" style="width: 2rem; height: 2rem;">
+                        <span class="visually-hidden">Wird geladen...</span>
+                    </div>
+                    <div class="text-muted small">Weiterleitung wird vorbereitet...</div>
+                </div>
+            </DxLoadingPanel>
+        </div>
+    </div>
 </div>
 
-<h2 class="mb-4 mt-4">Helpful Resources</h2>
+@code {
+    protected override async Task OnAfterRenderAsync(bool firstRender) {
+        if(!firstRender)
+            return;
 
-<ul>
-    <li><a class="helplink" href="https://demos.devexpress.com/blazor/ReportViewer/" target="_blank">Online Demos</a></li>
-    <li><a class="helplink" href="https://github.com/DevExpress-Examples/Reporting-Blazor-Getting-Started" target="_blank">Examples</a></li>
-    <li><a class="helplink" href="https://docs.devexpress.com/XtraReports/401676/web-reporting/blazor-reporting" target="_blank">Documentation</a></li>
-    <li><a class="helplink" href="https://www.devexpress.com/support/training/blazor/" target="_blank">Free Blazor Training Course</a></li>
-</ul>
+        await Task.Delay(1200);
+        NavigationManager.NavigateTo("/reportviewer");
+    }
+}