Add signature input and PDF export functionality

Introduced a signature input feature in `ReportViewer.razor` to allow users to draw a signature on a canvas and embed it into reports before exporting as a signed PDF.

- Added a canvas (`receiver-signature-pad`) and buttons for clearing, applying, and exporting signatures.
- Injected `IJSRuntime` for JavaScript interop and added methods for signature handling (`ClearSignatureAsync`, `ApplySignatureAsync`, `ExportSignedPdfAsync`).
- Embedded the signature as an image and label in the report's bottom margin.
- Added `receiver-signature.js` to manage the signature pad, including drawing, clearing, and exporting the signature as a Base64 image.
- Updated `index.html` to include the new JavaScript file.
- Displayed validation messages for missing or invalid signatures.

EnvelopeGenerator.ReceiverUI/wwwroot/index.html

@@ -62,6 +62,7 @@
         <a class="dismiss">X</a>
     </div>
     <script src="_content/DevExpress.Blazor.Resources/js/preload-script.js"></script>
+    <script src="js/receiver-signature.js"></script>
     <script src="_framework/blazor.webassembly.js"></script>
 </body>