Refactor services to remove ApiOptions dependency

Simplified `DocReceiverElementService` and `EnvelopeService` by removing the `ApiOptions` dependency. Updated constructors to eliminate the `IOptions<ApiOptions>` parameter and switched to using relative URLs directly for API requests.

Removed unused `BaseUrl` and `UsePredefinedReports` properties from `ApiOptions`. Cleaned up redundant fields, constructor logic, and unused `using` directives in affected services.

Registered `EnvelopeService` in `Program.cs` with `AddScoped`. These changes improve maintainability, reduce configuration overhead, and make the services more self-contained.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Options/ApiOptions.cs

@@ -4,7 +4,5 @@ public class ApiOptions
 {
     public const string SectionName = "Api";
 
-    public string BaseUrl { get; set; } = string.Empty;
-
     public bool UsePredefinedReports { get; set; } = false;
 }