Refactor envelope sender handling and update namespaces

Changed the namespace for `EnvelopeQuery` and updated the `Sender` parameter type to `SenderQuery?`. Removed `UserQuery.cs` as it is no longer needed. Introduced `SenderQuery.cs` with properties for sender details and added XML documentation for clarity.

EnvelopeGenerator.Application/Envelopes/SenderQuery.cs

@@ -0,0 +1,11 @@
+namespace EnvelopeGenerator.Application.Envelopes;
+
+/// <summary>
+/// Repräsentiert eine Abfrage für einen Absender.
+/// </summary>
+/// <param name="Id">Die eindeutige Kennung des Absenders.</param>
+/// <param name="Username">Der Benutzername des Absenders.</param>
+/// <param name="Email">Die E-Mail-Adresse des Absenders.</param>
+public record SenderQuery(int? Id = null, string? Username = null, string? Email = null)
+{
+}