Refine authorization and rendering mechanisms

Updated `EnvelopeSenderPage.razor` to replace the `[Authorize]`
attribute with the `@rendermode InteractiveWebAssembly` directive,
indicating a shift in how authorization or rendering is handled.

Modified the `Check` method in `AuthController.cs` to specify
`AuthenticationSchemes = AuthScheme.Sender` in the `[Authorize]`
attribute, enforcing a more specific authentication scheme for
this endpoint.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/EnvelopeSenderPage.razor

@@ -1,6 +1,5 @@
 @page "/sender"
-@attribute [Microsoft.AspNetCore.Authorization.Authorize(Policy = "Sender")]
-
+@rendermode InteractiveWebAssembly
 @using System.Text.Json
 @using EnvelopeGenerator.Domain.Constants
 @using EnvelopeGenerator.Server.Client.Models