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).
- Added a commented-out block for "Home" and "Document Viewer (JS-Based)" navigation links.
- Updated the "reportviewer" link text to "Empfänger-UI."
- Updated the "reportdesigner" link text to "Umschlag-UI."
