Improve PDF toolbar layout and rendering logic

Updated `envelope-viewer.css` to enhance the layout and responsiveness of the PDF toolbar:
- Adjusted padding, gap, width, and alignment for better usability.
- Improved zoom section and slider styles for flexibility and consistency.

Enhanced `pdf-viewer.js` to handle concurrent rendering tasks:
- Added checks to prevent overlapping render tasks on the same canvas.
- Implemented error handling for rendering operations to ensure stability.

These changes improve the user experience and robustness of the PDF viewer.

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

@@ -303,6 +303,15 @@ window.pdfViewer = {
                 return;
             }
 
+            // Check if canvas is already being used by a render task
+            if (canvas._renderTask) {
+                try {
+                    await canvas._renderTask;
+                } catch (e) {
+                    // Ignore cancellation errors
+                }
+            }
+
             const page = await this.pdfDoc.getPage(pageNum);
             
             // High-quality rendering with HiDPI support (configurable)
@@ -333,7 +342,14 @@ window.pdfViewer = {
                 viewport: viewport
             };
 
-            await page.render(renderContext).promise;
+            // Store render task on canvas to track active renders
+            const renderTask = page.render(renderContext);
+            canvas._renderTask = renderTask.promise;
+            
+            await renderTask.promise;
+            
+            // Clear the task when done
+            delete canvas._renderTask;
         } catch (error) {
             console.error('Error rendering thumbnail', pageNum, ':', error);
         }