Add envelope access check and update thumbnail width

Updated `MaxThumbnailWidth` to 400 in `EnvelopeViewer.razor`.
Added an authentication check using `AuthService.CheckEnvelopeAccessAsync`
to ensure users have access to the envelope. Redirects unauthorized
users to a login page with the `EnvelopeKey` in the URL. The check
is performed before fetching the document via `DocumentService`.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -435,6 +435,13 @@ const int MaxThumbnailWidth = 400;
             return;
         }
 
+        // Check authentication
+        var hasAccess = await AuthService.CheckEnvelopeAccessAsync(EnvelopeKey);
+        if (!hasAccess) {
+            Navigation.NavigateTo($"/login/{Uri.EscapeDataString(EnvelopeKey)}");
+            return;
+        }
+
         try {
             var (pdfBytes, statusCode) = await DocumentService.GetDocumentAsync(EnvelopeKey);