Updated the login route from `/login/{EnvelopeKey}` to
`/envelope/login/{EnvelopeKey}` across the application, including
navigation paths in `EnvelopeReceiver.razor` and the `@page` directive
in `LoginReceiver.razor`.
Refactored `_dotNetRef` in `EnvelopeReceiver.razor` to use
`DotNetObjectReference<enveloperece>?` instead of
`DotNetObjectReference<EnvelopeReceiver>?` to reflect a change in the
referenced type.
