Replaced custom zoom controls in `EnvelopeReceiverPage.razor` with the built-in zoom functionality of the `DevExpress.Blazor.PdfViewer` component (`DxPdfViewer`).
- Removed custom zoom buttons, slider, and JavaScript zoom logic.
- Introduced `_viewerZoomLevel` to align with `DxPdfViewer.ZoomLevel`.
- Synchronized zoom state using `ZoomLevelChanged` to update `_currentZoom`.
- Updated overlay redraw logic to react to viewer zoom changes.
- Modified `FitToWidth` and `SetZoom` methods to work with `DxPdfViewer`.
- Updated documentation to reflect integration decisions and findings.
These changes simplify zoom handling, reduce UI redundancy, and ensure proper synchronization between the viewer and overlay logic.