Add EnvelopeService to DI container in Program.cs

Registered EnvelopeService with a scoped lifetime in the
dependency injection container by adding
`builder.Services.AddScoped<EnvelopeService>();` to Program.cs.
This ensures a new instance is created per HTTP request.

EnvelopeGenerator.ReceiverUI/Program.cs

@@ -23,6 +23,7 @@ builder.Services.AddScoped<EnvelopeReceiverService>();
 builder.Services.AddScoped<SignatureService>();
 builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
+builder.Services.AddScoped<EnvelopeService>();
 
 builder.Services.AddDevExpressWebAssemblyBlazorReportViewer();
 builder.Services.AddDevExpressWebAssemblyBlazorPdfViewer();