Remove unsupported ZoomLevelChanged from DxPdfViewer

Removed the `ZoomLevelChanged` parameter from the `DxPdfViewer`
component in `EnvelopeReceiverPage.razor` due to lack of support
in the installed `DevExpress.Blazor.PdfViewer` package version
`25.2.3`. This prevents runtime exceptions caused by the use of
an unsupported parameter.

Deleted the `OnViewerZoomLevelChanged` method, as it is no longer
needed. Updated `RECEIVER_PDF_VIEWER_CONTEXT.md` to reflect the
limitations of the installed package and adjusted the recovery
plan to bind zoom state directly to `DxPdfViewer.ZoomLevel`.

Simplified zoom handling by removing custom JavaScript logic for
`ctrl+wheel` zoom and retaining the overlay redraw pipeline.
Confirmed that the built-in DevExpress zoom UI now works without
runtime errors, and custom zoom duplication has been eliminated.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeReceiverPage.razor

@@ -334,7 +334,6 @@
                                              CssClass="envelope-dx-pdf-viewer"
                                              DocumentContent="@_pdfDocumentContent"
                                              ZoomLevel="@_viewerZoomLevel"
-                                             ZoomLevelChanged="OnViewerZoomLevelChanged"
                                              IsSinglePagePreview="true" />
                             }
                             <div id="pdf-signature-layer" class="pdf-signature-layer pdf-signature-layer--dx"></div>
@@ -749,21 +748,6 @@
         await SetZoom(requestedZoom);
     }
 
-    async Task OnViewerZoomLevelChanged(double newZoomLevel)
-    {
-        _viewerZoomLevel = newZoomLevel;
-
-        if (newZoomLevel > 0)
-        {
-            _currentZoom = (int)Math.Round(newZoomLevel * 100, MidpointRounding.AwayFromZero);
-        }
-
-        await JSRuntime.InvokeVoidAsync("pdfViewer.setViewState", _currentPage, _currentZoom);
-        await Task.Delay(150);
-        await RenderSignatureButtonsAsync();
-        await InvokeAsync(StateHasChanged);
-    }
-
     async Task NextPage()
     {
         if (_currentPage >= _totalPages)