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 {
+
+}

EnvelopeGenerator.ReceiverUI/Pages/LoginSender.razor

@@ -1,4 +1,4 @@
-@page "/login"
+@page "/sender/login"
 @using EnvelopeGenerator.ReceiverUI.Services
 @inject AuthService AuthService
 @inject NavigationManager Navigation
@@ -160,7 +160,7 @@
         var result = await AuthService.LoginSenderAsync(Username.Trim(), Password.Trim());
 
         if (result == SenderLoginResult.Success) {
-            Navigation.NavigateTo("/", forceLoad: true);
+            Navigation.NavigateTo("/sender", forceLoad: true);
             return;
         }