Make ToggleThumbnails method asynchronous

Refactor the `ToggleThumbnails` method to be asynchronous (`async Task`) to support asynchronous operations. Add logic to re-render thumbnails when toggled on, including forcing a UI update, waiting for DOM rendering, and invoking `RenderThumbnailsAsync`. These changes improve the user experience by ensuring thumbnails are properly updated when displayed.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -318,8 +318,15 @@ protected override async Task OnInitializedAsync() {
         _currentZoom = (int)(scale * 100);
     }
 
-    void ToggleThumbnails() {
+    async Task ToggleThumbnails() {
         _showThumbnails = !_showThumbnails;
+        
+        // Re-render thumbnails when showing them
+        if (_showThumbnails && _pdfLoaded) {
+            await InvokeAsync(StateHasChanged); // Force UI update first
+            await Task.Delay(150); // Wait for DOM to render canvas elements
+            await RenderThumbnailsAsync();
+        }
     }
 
     async Task GoToPageFromThumbnail(int pageNum) {