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/Program.cs

@@ -32,6 +32,7 @@ builder.Services.AddScoped<EnvelopeReceiverService>();
 builder.Services.AddScoped<SignatureService>();
 builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
+builder.Services.AddScoped<EnvelopeService>();
 
 // DevExpress WASM
 builder.Services.AddDevExpressWebAssemblyBlazorPdfViewer();