Introduced a logout feature to the `ReceiverPage` component:
- Added a logout button to the toolbar with `_isLoggingOut` state.
- Implemented `LogoutAsync` method to handle logout logic, including navigation and error handling.
- Injected `IMediator` and `ILogger<ReceiverPage>` for logging and MediatR functionality.
- Added a check in `OnInitializedAsync` to redirect users to `/envelope/signed-result` if the envelope is already signed.
- Updated namespaces to include MediatR and custom extensions.
These changes improve user experience and enhance the component's functionality.
TekH
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