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/css/envelope-viewer.css

@@ -434,9 +434,14 @@ body.resizing {
     justify-content: flex-start;
 }
 
-.pdf-canvas {
+.pdf-page-container {
+    position: relative;
     display: inline-block;
     box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
+}
+
+.pdf-canvas {
+    display: block;
     vertical-align: top;
     image-rendering: -webkit-optimize-contrast;
     image-rendering: crisp-edges;
@@ -447,6 +452,34 @@ body.resizing {
     opacity: 0;
 }
 
+.pdf-text-layer {
+    position: absolute;
+    left: 0;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    overflow: hidden;
+    opacity: 1;
+    line-height: 1.0;
+    pointer-events: auto;
+}
+
+.pdf-text-layer > span {
+    color: transparent;
+    position: absolute;
+    white-space: pre;
+    cursor: text;
+    transform-origin: 0% 0%;
+}
+
+.pdf-text-layer ::selection {
+    background: rgba(126, 34, 206, 0.3);
+}
+
+.pdf-text-layer ::-moz-selection {
+    background: rgba(126, 34, 206, 0.3);
+}
+
 .error-container {
     display: flex;
     align-items: center;