Integrate DxPdfViewer and remove custom zoom controls

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.

RECEIVER_PDF_VIEWER_CONTEXT.md

@@ -939,6 +939,77 @@ That is the safest first vertical slice because:
 - it reduces uncertainty in overlay scaling
 - it does not yet require full signature flow completion
 
+### 9. Verified DevExpress API findings and current progress
+
+The following points are now verified from DevExpress documentation for `DxPdfViewer`:
+
+- `ZoomLevel` is a real component parameter
+- positive `ZoomLevel` values are interpreted as percentages
+- `ZoomLevelChanged` is supported and should be used to synchronize viewer zoom with page state
+- `CustomizeToolbar` can clear all built-in toolbar items through `ToolbarModel.AllItems.Clear()`
+- `ActivePageIndex` is read-only information and is not a page-navigation parameter
+
+### 10. Recovery progress update
+
+The first functional recovery step is zoom restoration.
+
+Implemented direction:
+
+- bind custom receiver toolbar zoom controls to `DxPdfViewer.ZoomLevel`
+- synchronize zoom changes back into `_currentZoom` through `ZoomLevelChanged`
+- remove competing custom JS ctrl+wheel zoom logic
+- keep overlay redraw pipeline after confirmed zoom changes
+
+Expected outcome after this step:
+
+- toolbar zoom buttons visibly affect the DevExpress viewer
+- zoom slider visibly affects the DevExpress viewer
+- overlay refresh is triggered from real viewer zoom state instead of guessed zoom state
+
+### 11. Additional findings after first zoom integration attempt
+
+After the first live integration attempt against the installed `DevExpress.Blazor.PdfViewer` package version `25.2.3`, one important runtime behavior was confirmed:
+
+- the `DxPdfViewer.ZoomLevel` value must be treated as a zoom factor for normal positive values in the live UI flow used here
+- in practice, `1.5` corresponds to `150%`
+- using `150` as the bound value causes the DevExpress viewer UI to display `15000%`
+
+This means the receiver page must keep two different zoom representations:
+
+- `_currentZoom` = receiver custom workflow percentage view, for example `150`
+- `_viewerZoomLevel` = DevExpress viewer value, for example `1.5`
+
+### 12. Current UI decision for zoom controls
+
+The custom toolbar zoom section in `pdf-toolbar__zoom-section` is no longer considered desirable.
+
+Current decision:
+
+- remove the custom zoom buttons and slider from the receiver toolbar
+- rely on the built-in DevExpress PDF Viewer zoom UI instead
+- keep `ZoomLevelChanged` synchronization so overlay redraw logic can still react to viewer zoom changes
+
+Reason:
+
+- DevExpress already provides zoom UX
+- duplicate zoom controls create confusing UX and unit mismatch risks
+- the built-in viewer zoom UI is a better source of truth for the current zoom state
+
+### 13. Current UI decision for thumbnails
+
+The custom thumbnail sidebar is still kept for now.
+
+Reason:
+
+- current receiver workflow depends on custom thumbnail shell behavior
+- width persistence and resizable splitter are already implemented in the custom sidebar
+- no verified built-in `DxPdfViewer` thumbnail sidebar integration surface has yet been confirmed from the currently inspected API surface
+
+So the current decision is:
+
+- keep custom thumbnail sidebar for now
+- revisit possible DevExpress-native thumbnail navigation later only if it supports the required receiver workflow behavior
+
 ---
 
 ## Files Most Likely Relevant For Future Work