Improve signature navigation and rendering stability

Enhanced signature navigation and rendering logic in `pdf-viewer.js`:
- Added `_renderLock` to prevent concurrent page renders.
- Refactored `renderPage` and `queueRenderPage` for stability.
- Updated `goToNextSignature` to support cross-page navigation.
- Filtered out applied signatures during rendering and navigation.
- Improved handling of applied signatures visibility per page.

Updated `EnvelopeViewer.razor`:
- Added `OnPageChangedBySignatureNav` to handle page changes triggered by signature navigation.

Improved code readability, added comments, and removed outdated logic to ensure smooth transitions and better user experience.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -616,6 +616,12 @@ const int MaxThumbnailWidth = 400;
         await UpdateSignatureCounterAsync();
     }
 
+    [JSInvokable]
+    public async Task OnPageChangedBySignatureNav(int newPage) {
+        _currentPage = newPage;
+        await RenderSignatureButtonsAsync();
+    }
+
     async Task UpdateSignatureCounterAsync() {
         try {
             var state = await JSRuntime.InvokeAsync<SignatureNavState>("pdfViewer.getSignatureNavState");