Migrate PDF.js to DevExpress DxPdfViewer

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`.

COPILOT_CONTEXT.md

@@ -185,11 +185,79 @@ For signature placeholders, the service:
 Current receiver viewer characteristics:
 - route: `/envelope/{EnvelopeKey}`
 - render mode: `InteractiveServer`
-- PDF rendering: `PDF.js`
+- PDF rendering: **Migration in progress from `PDF.js` to `DxPdfViewer`**
 - toolbar: page navigation, zoom, thumbnail toggle, signature navigation, signature reset
 - signature popup: `DxPopup`
 - thumbnail sidebar: resizable and stored in `localStorage`
 
+### ⚠️ CRITICAL: DevExpress DxPdfViewer Control Requirements
+
+**Verified API for installed `DevExpress.Blazor.PdfViewer` v25.2.3:**
+
+| Property | Access | Notes |
+|----------|--------|-------|
+| `DocumentContent` | `[Parameter]` GET/SET | Feed PDF as `byte[]` |
+| `ZoomLevel` | `[Parameter]` GET/SET | **Factor** (not percentage): `1.5` = 150% |
+| `IsSinglePagePreview` | `[Parameter]` GET/SET | Single page mode |
+| `CssClass` | `[Parameter]` GET/SET | CSS class |
+| `DocumentName` | `[Parameter]` GET/SET | Download filename |
+| `SizeMode` | `[Parameter]` GET/SET | `Small` / `Medium` / `Large` |
+| `PageCount` | Read-only GET | Total pages — **no JS call needed** |
+| `ActivePageIndex` | Read-only GET | Current page (0-based) — **cannot SET** |
+| `CustomizeToolbar` | Event | Only available toolbar event |
+
+**Does NOT exist in v25.2.3 — do NOT use:**
+- `GoToPageAsync()` ❌
+- `GoToNextPageAsync()` ❌
+- `ZoomAsync()` ❌
+- `PageNumberChanged` event ❌
+- `ZoomLevelChanged` event ❌
+- `ToolbarVisible` property ❌
+
+**Correct approach:**
+```razor
+<DxPdfViewer @ref="_pdfViewer"
+             DocumentContent="@_pdfDocumentContent"
+             ZoomLevel="@_viewerZoomLevel"
+             IsSinglePagePreview="true"
+             CustomizeToolbar="OnCustomizeToolbar" />
+```
+
+```csharp
+// ZoomLevel: always divide by 100 (factor, not percentage)
+_viewerZoomLevel = _currentZoom / 100d;  // 150 -> 1.5
+
+// PageCount: read directly, no JS needed
+_totalPages = _pdfViewer.PageCount;
+
+// Page navigation: only via CustomizeToolbar buttons
+protected void OnCustomizeToolbar(ToolbarModel toolbarModel)
+{
+    toolbarModel.AllItems.Clear();
+    var nextButton = new ToolbarItem
+    {
+        IconCssClass = "dx-icon-chevronnext",
+        Enabled = _currentPage < _totalPages,
+        Click = async (args) =>
+        {
+            _currentPage++;
+            _viewerZoomLevel = _currentZoom / 100d;
+            await InvokeAsync(StateHasChanged);
+            await RenderSignatureButtonsAsync();
+        }
+    };
+    toolbarModel.AllItems.Add(nextButton);
+}
+```
+
+**JavaScript role after migration:**
+- Overlay geometry calculations only
+- Thumbnail rendering via PDF.js helper
+- Custom UI interactions (sidebar resize, signature canvas)
+- **NOT** for controlling DxPdfViewer page or zoom
+
+See `DEVEXPRESS_V25_LIMITATIONS.md` for complete verified API reference.
+
 ### JS Assets
 - `EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/pdf-viewer.js`
 - `EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/receiver-signature.js`