Add sender authentication check to EnvelopeSenderPage

Added an authentication check in `EnvelopeSenderPage.razor` to verify sender access before loading envelopes. Redirects unauthorized users to the sender login page.

Introduced `CheckSenderAsync` in `AuthService` to validate sender tokens via the `/api/auth/check` endpoint. Updated `OnInitializedAsync` to use this method, enhancing security by ensuring only authorized users can access envelope-related functionality.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeSenderPage.razor

@@ -322,6 +322,13 @@
 
     protected override async Task OnInitializedAsync()
     {
+        var hasAccess = await AuthService.CheckSenderAsync();
+        if (!hasAccess)
+        {
+            Navigation.NavigateTo($"/sender/login");
+            return;
+        }
+
         await LoadEnvelopesAsync();
     }