Add PDF thumbnail sidebar to EnvelopeViewer

Introduced a PDF thumbnail sidebar in `EnvelopeViewer.razor` to enhance navigation between pages. Added a toggle button to show/hide the sidebar and implemented dynamic thumbnail rendering for all pages.

Updated `envelope-viewer.css` with styles for the sidebar, including hover/active states, transitions, and mobile responsiveness.

Enhanced `pdf-viewer.js` with a `renderThumbnail` method to render page previews on canvas elements. Added error handling for non-critical thumbnail rendering issues.

Improved user experience by providing an intuitive way to preview and navigate PDF pages.

EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js

@@ -233,6 +233,31 @@ window.pdfViewer = {
         }
     },
 
+    async renderThumbnail(pageNum, canvasId) {
+        if (!this.pdfDoc) return;
+
+        try {
+            const canvas = document.getElementById(canvasId);
+            if (!canvas) return;
+
+            const page = await this.pdfDoc.getPage(pageNum);
+            const viewport = page.getViewport({ scale: 0.2 });
+            
+            canvas.height = viewport.height;
+            canvas.width = viewport.width;
+
+            const ctx = canvas.getContext('2d');
+            const renderContext = {
+                canvasContext: ctx,
+                viewport: viewport
+            };
+
+            await page.render(renderContext).promise;
+        } catch (error) {
+            // Thumbnail rendering errors are non-critical
+        }
+    },
+
     getCurrentPage() {
         return this.pageNum;
     },