Refactor PDF editor UI and improve PDF loading logic

Simplify and restructure conditional logic in `SenderPdfImageEditor.razor` for better readability and maintainability. Merge redundant `else` blocks and reorganize rendering logic for page previews and fields.

Enhance PDF loading and page preview generation in `SenderEditorPage.razor`. Add error handling for failed page rendering, log warnings, and ensure consistent `_pageCount` initialization based on available data.

These changes improve code clarity, robustness, and maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Pages/SenderEditorPage.razor

@@ -826,11 +826,31 @@
 
             _originalPdfBytes = document?.ByteData;
             _pdfLoaded = _originalPdfBytes is { Length: > 0 };
-            _pagePreviews = [];
-            _pageCount = _pagePreviews.Count > 0
-                ? _pagePreviews.Count
-                : EstimatePdfPageCount(_originalPdfBytes)
-                    ?? Math.Max(1, _signatureFields.Select(f => f.Page).DefaultIfEmpty(1).Max());
+
+            if (_pdfLoaded)
+            {
+                try
+                {
+                    _pagePreviews = (await PdfRenderService.RenderPagesAsync(_originalPdfBytes!))
+                        .OrderBy(p => p.PageNumber)
+                        .ToList();
+                }
+                catch (Exception renderEx)
+                {
+                    Logger.LogWarning(renderEx, "Failed to render PDF page previews for envelope {EnvelopeId}. Falling back to native preview.", envelopeId);
+                    _pagePreviews = [];
+                }
+
+                _pageCount = _pagePreviews.Count > 0
+                    ? _pagePreviews.Count
+                    : 1;
+            }
+            else
+            {
+                _pagePreviews = [];
+                _pageCount = Math.Max(1, _signatureFields.Select(f => f.Page).DefaultIfEmpty(1).Max());
+            }
+
             _fileName = $"Envelope-{envelopeId}.pdf";
             _pendingReceiverForPlacement = null;
             _loadedEnvelopeId = envelopeId;