feat: add SenderId() extension to ClaimsPrincipal

Extracts the authenticated sender's integer ID from the 'sub' /
NameIdentifier claim. Mirrors the existing ReceiverId() helper and is
required by GetDocumentOfSenderAsync.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Extensions/ReceiverClaimExtensions.cs

@@ -96,6 +96,21 @@ public static class ReceiverClaimExtensions
             throw new InvalidOperationException($"Claim '{EnvelopeClaimNames.ReceiverId}' is not a valid integer.");
     }
 
+    /// <summary>
+    /// Gets the authenticated sender identifier from the 'sub' claim.
+    /// </summary>
+    /// <param name="user"></param>
+    /// <returns></returns>
+    /// <exception cref="InvalidOperationException"></exception>
+    public static int SenderId(this ClaimsPrincipal user)
+    {
+        var senderIdStr = user.GetRequiredClaimValue(JwtRegisteredClaimNames.Sub, ClaimTypes.NameIdentifier);
+        if (int.TryParse(senderIdStr, out var senderId))
+            return senderId;
+        else
+            throw new InvalidOperationException($"Claim '{JwtRegisteredClaimNames.Sub}' is not a valid integer.");
+    }
+
     /// <summary>
     /// 
     /// </summary>