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).
