Update routes and navigation links for sender/receiver UI

Updated the `@page` directives in `ReportDesigner.razor` and
`ReportViewer.razor` to change routes from `/reportdesigner`
to `/sender` and `/reportviewer` to `/receiver`, respectively.

Modified `NavMenu.razor` to update navigation links to reflect
the new routes. Updated the displayed text for the links to
"Empfänger-UI" (Receiver UI) and "Umschlag-UI" (Envelope UI).

EnvelopeGenerator.ReceiverUI/Pages/ReportDesigner.razor

@@ -1,4 +1,4 @@
-@page "/reportdesigner"
+@page "/sender"
 @using DevExpress.DataAccess.Json;
 @using EnvelopeGenerator.ReceiverUI.Services;
 

EnvelopeGenerator.ReceiverUI/Pages/ReportViewer.razor

@@ -1,4 +1,4 @@
-@page "/reportviewer/"
+@page "/receiver"
 @using System.Drawing
 @using DevExpress.Drawing
 @using DevExpress.Utils

EnvelopeGenerator.ReceiverUI/Shared/NavMenu.razor

@@ -22,12 +22,12 @@
         </div>
         *@
         <div class="nav-item px-3">
-            <NavLink class="nav-link" href="reportviewer">
+            <NavLink class="nav-link" href="receiver">
                 <span class="oi oi-plus" aria-hidden="true"></span> Empfänger-UI
             </NavLink>
         </div>
         <div class="nav-item px-3">
-            <NavLink class="nav-link" href="reportdesigner">
+            <NavLink class="nav-link" href="sender">
                 <span class="oi oi-plus" aria-hidden="true"></span> Umschlag-UI
             </NavLink>
         </div>