Add interactive signature buttons to PDF viewer

Introduced functionality to render interactive signature buttons on the PDF viewer. Added support for fetching and displaying signature data (`SignatureDto`) dynamically based on the current page.

- Added `@using` directives in `EnvelopeViewer.razor` for required namespaces.
- Introduced `_signatures` field to store signature data.
- Updated `OnInitializedAsync` to fetch and process signatures.
- Implemented `RenderSignatureButtonsAsync` to dynamically render buttons.
- Added `[JSInvokable]` method `OnSignatureButtonClick` for button events.
- Updated CSS to style `pdf-signature-layer` and `signature-button`.
- Enhanced `pdf-viewer.js` with methods to render and clear buttons.
- Ensured buttons respond to zoom and page navigation changes.
- Added error handling and logging for signature rendering.

These changes improve user interaction by enabling signing functionality directly on the PDF viewer.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -1,4 +1,6 @@
 @page "/envelope/{EnvelopeKey}"
+@using EnvelopeGenerator.ReceiverUI.Models
+@using EnvelopeGenerator.ReceiverUI.Models.Constants
 @using EnvelopeGenerator.ReceiverUI.Services
 @using Microsoft.Extensions.Options
 @using EnvelopeGenerator.ReceiverUI.Options
@@ -155,6 +157,7 @@
                         <div class="pdf-page-container">
                             <canvas id="pdf-canvas" class="pdf-canvas"></canvas>
                             <div id="pdf-text-layer" class="pdf-text-layer"></div>
+                            <div id="pdf-signature-layer" class="pdf-signature-layer"></div>
                         </div>
                     </div>
                 </div>
@@ -186,6 +189,7 @@ int _totalPages = 0;
 int _currentZoom = 150;
 bool _showThumbnails = true;
 DotNetObjectReference<EnvelopeViewer>? _dotNetRef;
+IReadOnlyList<SignatureDto> _signatures = [];
 
 // Resizable splitter state
 int _thumbnailWidth = 260;
@@ -195,7 +199,7 @@ int _resizeStartWidth = 0;
 const int MinThumbnailWidth = 150;
 const int MaxThumbnailWidth = 400;
 
-protected override async Task OnInitializedAsync() {
+    protected override async Task OnInitializedAsync() {
         if (string.IsNullOrWhiteSpace(EnvelopeKey)) {
             _errorMessage = "Envelope-Schlüssel fehlt.";
             _isLoading = false;
@@ -213,8 +217,9 @@ protected override async Task OnInitializedAsync() {
             }
 
             var signatures = await SignatureService.GetAsync(EnvelopeKey);
+            _signatures = signatures.Convert(UnitOfLength.Point);
 
-            await JSRuntime.InvokeVoidAsync("console.log", signatures);
+            await JSRuntime.InvokeVoidAsync("console.log", "Loaded signatures:", _signatures);
 
         } catch (Exception ex) {
             _errorMessage = $"Fehler: {ex.Message}";
@@ -269,11 +274,15 @@ protected override async Task OnInitializedAsync() {
                     // Attach resize listeners
                     await JSRuntime.InvokeVoidAsync("pdfViewer.attachResizeListeners", _dotNetRef);
                     
+                    
                     await InvokeAsync(StateHasChanged);
                     
                     // Wait for DOM to be ready, then render thumbnails
                     await Task.Delay(100);
                     await RenderThumbnailsAsync();
+                    
+                    // Render signature buttons
+                    await RenderSignatureButtonsAsync();
                 }
             } catch (Exception ex) {
                 _errorMessage = $"PDF.js Fehler: {ex.Message}";
@@ -287,17 +296,23 @@ protected override async Task OnInitializedAsync() {
     {
         _currentZoom = (int)(scale * 100);
         await InvokeAsync(StateHasChanged);
+        
+        // Re-render signature buttons when zoom changes
+        await Task.Delay(100);
+        await RenderSignatureButtonsAsync();
     }
 
     async Task NextPage() {
         if (await JSRuntime.InvokeAsync<bool>("pdfViewer.nextPage")) {
             _currentPage = await JSRuntime.InvokeAsync<int>("pdfViewer.getCurrentPage");
+            await RenderSignatureButtonsAsync();
         }
     }
 
     async Task PreviousPage() {
         if (await JSRuntime.InvokeAsync<bool>("pdfViewer.previousPage")) {
             _currentPage = await JSRuntime.InvokeAsync<int>("pdfViewer.getCurrentPage");
+            await RenderSignatureButtonsAsync();
         }
     }
 
@@ -355,9 +370,25 @@ protected override async Task OnInitializedAsync() {
     async Task GoToPageFromThumbnail(int pageNum) {
         if (await JSRuntime.InvokeAsync<bool>("pdfViewer.goToPage", pageNum)) {
             _currentPage = pageNum;
+            await RenderSignatureButtonsAsync();
         }
     }
 
+    async Task RenderSignatureButtonsAsync() {
+        if (_signatures.Count == 0 || !_pdfLoaded) return;
+
+        try {
+            await JSRuntime.InvokeVoidAsync("pdfViewer.renderSignatureButtons", _signatures, _currentPage, _dotNetRef);
+        } catch (Exception ex) {
+            System.Diagnostics.Debug.WriteLine($"Signature button rendering error: {ex.Message}");
+        }
+    }
+
+    [JSInvokable]
+    public void OnSignatureButtonClick(int signatureId) {
+        Console.WriteLine($"Signature #{signatureId} signed");
+    }
+
     async Task RenderThumbnailsAsync() {
         try {
             var delay = PdfViewerOptions.Value.ThumbnailRenderDelay;