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.
Introduced the `EnvelopeDto` class to represent envelope data with JSON property mappings. Added the `EnvelopeService` class to handle API interactions, including fetching envelopes with optional filters and query string construction using `Microsoft.AspNetCore.WebUtilities`. Updated the project file to include the required package reference for query string manipulation.
