Update EnvelopeSigned view model and title logic

Set model to EnvelopeReceiverDto and import required types. Update ViewData["Title"] to use DocSigned or DocConfirmed based on Envelope.IsReadAndConfirm() result.

EnvelopeGenerator.Web/Views/Envelope/EnvelopeSigned.cshtml

@@ -1,5 +1,10 @@
 @{
-    ViewData["Title"] = _localizer.DocSigned();
+    @using EnvelopeGenerator.Application.Common.Dto.EnvelopeReceiver;
+    @using EnvelopeGenerator.Domain.Interfaces;
+    @model EnvelopeReceiverDto;
+    ViewData["Title"] = Model!.Envelope!.IsReadAndConfirm()
+        ? _localizer.DocSigned()
+        : _localizer.DocConfirmed();
 }
 <div class="page container p-5">
     <header class="text-center">