Refactor EnvelopeDto and add EnvelopeStatus enum

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.

EnvelopeGenerator.ReceiverUI/Models/EnvelopeDto.cs

@@ -20,5 +20,20 @@ public class EnvelopeDto
     public byte[]? DocResult { get; set; }
 
     [JsonPropertyName("envelopeReceivers")]
-    public List<EnvelopeReceiverDto> EnvelopeReceivers { get; set; } = new();
+    public List<EnvelopeReceiverSimpleDto> EnvelopeReceivers { get; set; } = new();
+}
+
+/// <summary>
+/// Simplified receiver model for envelope list display
+/// </summary>
+public class EnvelopeReceiverSimpleDto
+{
+    [JsonPropertyName("name")]
+    public string? Name { get; set; }
+
+    [JsonPropertyName("email")]
+    public string? Email { get; set; }
+
+    [JsonPropertyName("signed")]
+    public bool Signed { get; set; }
 }