Refactor PDF editor with custom rendering

Replaced `DxPdfViewer` with a custom PDF rendering solution in `SenderPdfImageEditor` to improve flexibility and control. Added support for displaying placeholder messages and interactive signature fields. Introduced the `SenderPdfFieldOverlay` record to encapsulate field data.

Updated `SenderEditorPage` logic to manage page counts, image URLs, and field overlays. Added a method to estimate PDF page counts. Refactored field removal logic to use index-based operations.

Enhanced styling in `envelope-viewer.css` for improved layout and responsiveness. Updated localization files with new strings for page labels and placeholders. Adjusted JavaScript selectors in `envelope-editor.js` to support the new rendering approach.

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

@@ -276,10 +276,13 @@
         }
         else
         {
-            <SenderPdfImageEditor PdfBytes="@_pdfBytes"
-                                  IsPlacementMode="@(_pendingReceiverForPlacement is not null)"
-                                  ViewerCssClass="@ViewerCssClass"
-                                  EditorClick="OnPdfAreaClickAsync" />
+            <SenderPdfImageEditor IsPlacementMode="@(_pendingReceiverForPlacement is not null)"
+                                  PageCount="@_pageCount"
+                                  PageImageDataUrls="@_pageImageDataUrls"
+                                  PdfDataUrl="@PdfDataUrl"
+                                  Fields="@FieldOverlays"
+                                  EditorClick="OnPdfAreaClickAsync"
+                                  RemoveField="OnRemoveFieldByIndexAsync" />
         }
     </div>
 </div>
@@ -459,15 +462,16 @@
     const double SigWidthPt = 1.77 * 72;   // 127.44 pt
     const double SigHeightPt = 1.96 * 72;   // 141.12 pt
 
-    // CssClass for DxPdfViewer — used by JS to locate page elements
+    // CssClass for JS click coordinate helper
     const string ViewerCssClass = "sender-editor-pdf-viewer";
 
     // ── State ──
     bool _pdfLoaded = false;
     string _fileName = string.Empty;
     string? _errorMessage;
-    byte[]? _pdfBytes;          // Current rendered PDF (original + placeholders burned in)
     byte[]? _originalPdfBytes;  // Pristine upload — never modified, used as base for redraw
+    int _pageCount;
+    Dictionary<int, string> _pageImageDataUrls = [];
     int? _loadedEnvelopeId;
     int? _editingEnvelopeId;
     bool _isLoadingEnvelopeForEdit;
@@ -475,6 +479,14 @@
     List<SignatureFieldDraft> _signatureFields = [];
     ReceiverDraft? _pendingReceiverForPlacement; // Set when user clicks "Signatur hinzufügen"
 
+    IReadOnlyList<SenderPdfFieldOverlay> FieldOverlays => _signatureFields
+        .Select((f, i) => new SenderPdfFieldOverlay(i, f.Page, f.XPt, f.YPt, f.ReceiverName, f.Color))
+        .ToList();
+
+    string? PdfDataUrl => _originalPdfBytes is { Length: > 0 }
+        ? $"data:application/pdf;base64,{Convert.ToBase64String(_originalPdfBytes)}"
+        : null;
+
     // ── Save state ──
     bool _isSaving = false;
     bool _savePopupVisible = false;
@@ -582,7 +594,9 @@
 
             _originalPdfBytes = document?.ByteData;
             _pdfLoaded = _originalPdfBytes is { Length: > 0 };
-            _pdfBytes = _pdfLoaded ? _originalPdfBytes : null;
+            _pageCount = EstimatePdfPageCount(_originalPdfBytes) ??
+                Math.Max(1, _signatureFields.Select(f => f.Page).DefaultIfEmpty(1).Max());
+            _pageImageDataUrls.Clear();
             _fileName = $"Envelope-{envelopeId}.pdf";
             _pendingReceiverForPlacement = null;
             _loadedEnvelopeId = envelopeId;
@@ -624,12 +638,11 @@
             _originalPdfBytes = ms.ToArray();
             _fileName = file.Name;
             _pdfLoaded = true;
+            _pageCount = EstimatePdfPageCount(_originalPdfBytes) ?? 1;
+            _pageImageDataUrls.Clear();
             _signatureFields.Clear();
             _pendingReceiverForPlacement = null;
 
-            // Rendered PDF starts as the clean original (no placeholders yet)
-            _pdfBytes = _originalPdfBytes;
-
             Logger.LogInformation("PDF loaded: {Name} ({Size} bytes)", _fileName, _originalPdfBytes.Length);
         }
         catch (Exception ex)
@@ -693,19 +706,18 @@
         _signatureFields.Add(field);
         _pendingReceiverForPlacement = null;
 
-        // Keep original PDF bytes unchanged. Signature boxes are tracked in-memory.
-        _pdfBytes = _originalPdfBytes;
-
         Logger.LogInformation(
             "[SenderEditor] Field added: Page={Page} X={X:F1}pt Y={Y:F1}pt Receiver={Receiver}",
             page1Based, xPt, yPt, field.ReceiverName);
     }
 
     // ── Remove a single field ──
-    async Task RemoveFieldAsync(SignatureFieldDraft field)
+    async Task OnRemoveFieldByIndexAsync(int index)
     {
-        _signatureFields.Remove(field);
-        _pdfBytes = _originalPdfBytes;
+        if (index < 0 || index >= _signatureFields.Count)
+            return;
+
+        _signatureFields.RemoveAt(index);
         await Task.CompletedTask;
     }
 
@@ -714,10 +726,39 @@
     {
         _signatureFields.Clear();
         _pendingReceiverForPlacement = null;
-        _pdfBytes = _originalPdfBytes;
         await Task.CompletedTask;
     }
 
+    static int? EstimatePdfPageCount(byte[]? documentBytes)
+    {
+        if (documentBytes is null || documentBytes.Length == 0)
+            return null;
+
+        try
+        {
+            var text = System.Text.Encoding.ASCII.GetString(documentBytes);
+            const string marker = "/Type /Page";
+            var count = 0;
+            var index = 0;
+
+            while (true)
+            {
+                index = text.IndexOf(marker, index, StringComparison.Ordinal);
+                if (index < 0)
+                    break;
+
+                count++;
+                index += marker.Length;
+            }
+
+            return count > 0 ? count : null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
     void Cancel() => NavigationManager.NavigateTo("/sender");
 
     void GoToDashboard()