Add LogoutSenderAsync method to AuthService

A new asynchronous method `LogoutSenderAsync` was added to the `AuthService` class to handle sender user logout. The method sends a POST request to the `/api/auth/logout` endpoint and removes the authentication cookie. It accepts an optional `CancellationToken` parameter and returns a `bool` indicating the success of the operation. XML documentation comments were included to describe the method's functionality.

EnvelopeGenerator.ReceiverUI/Services/AuthService.cs

@@ -78,4 +78,16 @@ public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
             _ => SenderLoginResult.Error
         };
     }
+
+    /// <summary>
+    /// Logs out the sender user by removing the authentication cookie.
+    /// Calls POST /api/auth/logout.
+    /// </summary>
+    public async Task<bool> LogoutSenderAsync(CancellationToken cancel = default)
+    {
+        var response = await http.PostAsync(
+            $"{_api.BaseUrl}/api/auth/logout",
+            null, cancel);
+        return response.IsSuccessStatusCode;
+    }
 }