Refactor signature handling and add PDF signature support

Refactored `OnSignatureButtonClick` in `EnvelopeViewer.razor`:
- Converted to async and added null-check for `_capturedSignature`.
- Integrated `pdfViewer.applySignature` to apply signatures to PDFs.

Added `applySignature` method to `pdf-viewer.js`:
- Handles rendering of signatures with image, metadata, and styling.
- Follows German standards for signature formatting.
- Includes error handling for missing elements.

Introduced `escapeHtml` helper in `pdf-viewer.js` to prevent XSS.
Updated `MaxThumbnailWidth` in `EnvelopeViewer.razor` to 400.
Enhanced logging for better debugging during signature application.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -566,9 +566,19 @@ const int MaxThumbnailWidth = 400;
     }
 
     [JSInvokable]
-    public void OnSignatureButtonClick(int signatureId) {
-        Console.WriteLine($"Signature #{signatureId} clicked");
-        OpenSignaturePopup();
+    public async Task OnSignatureButtonClick(int signatureId) {
+        if (_capturedSignature == null) {
+            // No signature captured yet - should not happen as popup is shown on page load
+            return;
+        }
+
+        // Apply signature to PDF canvas
+        await JSRuntime.InvokeVoidAsync("pdfViewer.applySignature", 
+            signatureId, 
+            _capturedSignature.DataUrl,
+            _capturedSignature.FullName,
+            _capturedSignature.Position,
+            _capturedSignature.Place);
     }
 
     // Signature popup methods