Add logout functionality for envelope sessions

Introduced a "Logout" button in `ReportViewer.razor` that is displayed when an `EnvelopeKey` is present. The button includes a spinner to indicate progress during the logout process.

Added the `LogoutAsync` method in `ReportViewer.razor` to handle the logout operation, preventing multiple attempts and navigating the user to the login page upon success.

Implemented `LogoutEnvelopeReceiverAsync` in `AuthService` to send a POST request to remove the per-envelope receiver cookie. This ensures proper backend handling of the logout process.

Improved user experience by providing a responsive and clear logout mechanism for envelope-based sessions.

EnvelopeGenerator.ReceiverUI/Services/AuthService.cs

@@ -42,4 +42,16 @@ public class AuthService(HttpClient http, IOptions<ApiOptions> apiOptions)
             _ => EnvelopeLoginResult.Error
         };
     }
+
+    /// <summary>
+    /// Removes the per-envelope receiver cookie for the given envelope key.
+    /// Calls POST /api/auth/logout/envelope/{envelopeKey}.
+    /// </summary>
+    public async Task<bool> LogoutEnvelopeReceiverAsync(string envelopeKey, CancellationToken cancel = default)
+    {
+        var response = await http.PostAsync(
+            $"{_api.BaseUrl}/api/auth/logout/envelope/{Uri.EscapeDataString(envelopeKey)}",
+            null, cancel);
+        return response.IsSuccessStatusCode;
+    }
 }