Improve PDF viewer overlay synchronization

Refactor `EnvelopeSenderEditorPage.razor` to enhance the structure and behavior of the PDF editor wrapper:
- Add `class="pdf-editor-wrapper"` and update `overflow` to `auto`.
- Update `DxPdfViewer`'s `CssClass` to `sender-editor-pdf-viewer`.
- Introduce `OnAfterRenderAsync` to synchronize the overlay with the viewer.

Add new styles in `envelope-viewer.css` for better layout:
- Ensure `.pdf-editor-wrapper` and `.sender-editor-pdf-viewer` occupy full dimensions.
- Center and align content within the PDF viewer.

Enhance `envelope-editor.js` with `syncOverlayToPage`:
- Dynamically adjust overlay position and size relative to the viewer.
- Use `MutationObserver` and event listeners for real-time synchronization.
- Handle delayed rendering with scheduled sync attempts.

These changes improve overlay alignment, user experience, and code maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderEditorPage.razor

@@ -147,15 +147,15 @@
         else
         {
             @* PDF viewer + overlay wrapper *@
-            <div id="pdf-editor-wrapper"
-                 style="position: relative; width: 100%; height: 100%; overflow: hidden;">
+            <div id="pdf-editor-wrapper" class="pdf-editor-wrapper"
+                 style="position: relative; width: 100%; height: 100%; overflow: auto;">
 
                 @* DxPdfViewer — zoom fixed to 1.0 for reliable coordinate mapping *@
                 <DxPdfViewer @ref="_pdfViewer"
                              DocumentContent="@_pdfBytes"
                              ZoomLevel="1.0"
                              IsSinglePagePreview="false"
-                             CssClass="w-100 h-100" />
+                             CssClass="sender-editor-pdf-viewer" />
 
                 @* Transparent overlay for click capture (active only in placement mode) *@
                 <div id="pdf-editor-overlay"
@@ -336,6 +336,17 @@
             $"[SenderEditor] Total fields: {_signatureFields.Count}");
     }
 
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (!_pdfLoaded || _errorMessage is not null)
+            return;
+
+        await JSRuntime.InvokeVoidAsync(
+            "envelopeEditor.syncOverlayToPage",
+            "pdf-editor-wrapper",
+            "pdf-editor-overlay");
+    }
+
     // ── Models ──
     record SignatureFieldDraft(double XPt, double YPt, int Page, double DisplayX, double DisplayY);