Transitioned PDF rendering in `EnvelopeReceiverPage.razor`
from `PDF.js` to `DevExpress DxPdfViewer`. Updated code
and documentation to reflect the verified API of
`DevExpress.Blazor.PdfViewer` v25.2.3, addressing its
limitations (e.g., lack of `GoToPageAsync`, `PageNumberChanged`).
Implemented `CustomizeToolbar` for navigation/zoom controls
and manual state tracking for `_currentPage` and `_viewerZoomLevel`.
Replaced JavaScript interop for page count with the `PageCount`
property. Retained the custom thumbnail sidebar due to API
constraints.
Added temporary debug tools for DOM analysis and navigation
testing. Updated `TESTING_CHECKLIST.md` and added
`DEVEXPRESS_V25_LIMITATIONS.md` to document the new strategy,
API limitations, and testing scenarios. Cross-page signature
navigation implemented with state updates, though visible
page changes remain manual.
Prepared for future improvements while ensuring functional
migration to `DxPdfViewer`.
