Add task management for PDF rendering operations

Introduced `currentRenderTask` to track and manage rendering tasks, ensuring only one task is active at a time. Added logic to cancel ongoing tasks before starting new ones, preventing conflicts and redundant rendering.

Enhanced error handling to gracefully manage `RenderingCancelledException` and allow rendering of pending pages. Improved logging for better debugging and insights into the rendering process. Ensured clean state management by resetting `currentRenderTask` after task completion or cancellation.

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

@@ -1,13 +1,14 @@
 // PDF.js Viewer for Blazor WASM
 window.pdfViewer = {
-    pdfDoc: null,
-    pageNum: 1,
-    pageRendering: false,
-    pageNumPending: null,
-    scale: 1.5,
-    canvas: null,
-    ctx: null,
-    totalPages: 0,
+pdfDoc: null,
+pageNum: 1,
+pageRendering: false,
+pageNumPending: null,
+scale: 1.5,
+canvas: null,
+ctx: null,
+totalPages: 0,
+currentRenderTask: null,
 
     async initialize(canvasId, pdfDataUrl) {
         try {
@@ -90,10 +91,17 @@ window.pdfViewer = {
                 viewport: viewport
             };
 
-            await page.render(renderContext).promise;
+            if (this.currentRenderTask) {
+                console.log('Cancelling previous render task');
+                this.currentRenderTask.cancel();
+            }
+
+            this.currentRenderTask = page.render(renderContext);
+            await this.currentRenderTask.promise;
 
             console.log('Page rendered successfully');
 
+            this.currentRenderTask = null;
             this.pageRendering = false;
 
             if (this.pageNumPending !== null) {
@@ -101,7 +109,12 @@ window.pdfViewer = {
                 this.pageNumPending = null;
             }
         } catch (error) {
-            console.error('Error rendering page:', error);
+            if (error.name === 'RenderingCancelledException') {
+                console.log('Rendering cancelled, will render pending page');
+            } else {
+                console.error('Error rendering page:', error);
+            }
+            this.currentRenderTask = null;
             this.pageRendering = false;
         }
     },