Introduced `EnvelopeReceiverDto` and its nested models (`EnvelopeClientDto`, `EnvelopeSenderDto`, `DocumentClientDto`, `SignatureClientDto`, `ReceiverClientDto`) to represent client-side data for envelope receivers and their associated entities.
Added `EnvelopeReceiverService` to fetch `EnvelopeReceiverDto` from the API using `HttpClient`. Implemented error handling and JSON deserialization with `JsonSerializerDefaults.Web`. Updated necessary `using` directives.
