Update routes and add EnvelopeSender component

Added a new `EnvelopeSender.razor` component with the route `/sender`.
Updated the route for `LoginSender.razor` from `/login` to `/sender/login`.
Modified navigation logic in `LoginSender.razor` to redirect to
`/sender` after a successful login instead of the root route.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeSender.razor

@@ -0,0 +1,7 @@
+@page "/sender"
+
+<h3>EnvelopeSender</h3>
+
+@code {
+
+}