Updated the `EnvelopeDto` class to use a simplified receiver model (`EnvelopeReceiverSimpleDto`) for streamlined data handling. Added the `EnvelopeReceiverSimpleDto` class to represent basic receiver information (`Name`, `Email`, `Signed`).
Introduced the `EnvelopeStatus` enumeration in `EnvelopeStatus.cs` to define envelope lifecycle statuses, repurposed for the `ReceiverUI` context. Added `EnvelopeStatusExtensions` with `IsActive` and `IsCompleted` methods to evaluate envelope status states.