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/wwwroot/css/envelope-viewer.css

@@ -480,6 +480,34 @@ body.resizing {
     background: rgba(126, 34, 206, 0.3);
 }
 
+.pdf-signature-layer {
+    position: absolute;
+    left: 0;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    overflow: visible;
+    pointer-events: none;
+    z-index: 20;
+}
+
+.pdf-signature-layer .signature-button {
+    pointer-events: auto;
+}
+
+.signature-button {
+    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+}
+
+.signature-button:focus {
+    outline: 2px solid #7e22ce;
+    outline-offset: 2px;
+}
+
+.signature-button:active {
+    transform: scale(0.98);
+}
+
 .error-container {
     display: flex;
     align-items: center;