Add text layer support for PDF rendering and selection

Integrated PDF.js to enable text selection and copy-paste functionality in the PDF viewer. Updated `EnvelopeViewer.razor` to include the necessary scripts and styles, and modified the HTML structure to add a text layer container.

Enhanced `envelope-viewer.css` with styles for the text layer and optimized canvas rendering. Added a `renderTextLayer` method in `pdf-viewer.js` to extract and render text content from PDF pages. Updated the rendering process to overlay the text layer on the canvas.

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

@@ -176,6 +176,9 @@ window.pdfViewer = {
             this.currentRenderTask = page.render(renderContext);
             await this.currentRenderTask.promise;
 
+            // Render text layer for copy-paste functionality
+            await this.renderTextLayer(page, viewport, dpr);
+
             // Restore viewport center position AFTER rendering
             if (container && oldWidth > 0 && oldHeight > 0) {
                 const scaleX = this.canvas.width / oldWidth;
@@ -210,6 +213,41 @@ window.pdfViewer = {
         }
     },
 
+    async renderTextLayer(page, viewport, dpr) {
+        try {
+            const textLayerDiv = document.getElementById('pdf-text-layer');
+            if (!textLayerDiv) {
+                console.warn('Text layer div not found');
+                return;
+            }
+
+            // Clear previous text layer
+            textLayerDiv.innerHTML = '';
+            
+            // Set text layer dimensions to match canvas display size
+            textLayerDiv.style.width = `${viewport.width / dpr}px`;
+            textLayerDiv.style.height = `${viewport.height / dpr}px`;
+
+            // Get text content from PDF
+            const textContent = await page.getTextContent();
+            
+            // Create viewport for text layer (without DPR scaling for positioning)
+            const textViewport = page.getViewport({ scale: this.scale });
+
+            // Render text layer using PDF.js built-in function
+            const pdfjsLib = window.pdfjsLib;
+            await pdfjsLib.renderTextLayer({
+                textContentSource: textContent,
+                container: textLayerDiv,
+                viewport: textViewport,
+                textDivs: []
+            }).promise;
+
+        } catch (error) {
+            console.error('Error rendering text layer:', error);
+        }
+    },
+
     queueRenderPage(num) {
         if (this.pageRendering) {
             this.pageNumPending = num;