Refactor EnvelopeStatus to shared domain library

Moved the `EnvelopeStatus` enum and its extension methods
from `ReceiverUI.Models` to `Domain.Constants` to improve
code reuse and maintainability. Updated `EnvelopeSenderPage.razor`
to reference the new namespace.

EnvelopeGenerator.ReceiverUI/Models/EnvelopeStatus.cs

@@ -1,33 +0,0 @@
-namespace EnvelopeGenerator.ReceiverUI.Models;
-
-/// <summary>
-/// Envelope status enumeration (copied from Domain for ReceiverUI)
-/// </summary>
-public enum EnvelopeStatus
-{
-    Invalid = 0,
-    EnvelopeCreated = 1001,
-    EnvelopeSaved = 1002,
-    EnvelopeQueued = 1003,
-    EnvelopeSent = 1004,
-    EnvelopePartlySigned = 1005,
-    EnvelopeCompletelySigned = 1006,
-    EnvelopeReportCreated = 1007,
-    EnvelopeArchived = 1008,
-    EnvelopeDeleted = 1009,
-    EnvelopeRejected = 10007,
-    EnvelopeWithdrawn = 10009
-}
-
-public static class EnvelopeStatusExtensions
-{
-    public static bool IsActive(this EnvelopeStatus status)
-    {
-        return status >= EnvelopeStatus.EnvelopeCreated && status < EnvelopeStatus.EnvelopePartlySigned;
-    }
-
-    public static bool IsCompleted(this EnvelopeStatus status)
-    {
-        return status >= EnvelopeStatus.EnvelopeCompletelySigned && status <= EnvelopeStatus.EnvelopeWithdrawn;
-    }
-}