Improve PDF rendering quality and HiDPI support

Enhanced thumbnail rendering in `envelope-viewer.css` by adding `image-rendering` properties for better visual quality.

Updated `pdf-viewer.js` to support high-quality rendering with HiDPI support:
- Replaced fixed scale with dynamic scaling using `devicePixelRatio` (capped at 2x) and a base scale of 0.75.
- Adjusted canvas resolution to match scaled viewport dimensions.
- Removed inline canvas styles to delegate display size to CSS.
- Retained high-quality rendering settings for the canvas context.

These changes improve visual fidelity while maintaining performance.

EnvelopeGenerator.ReceiverUI/wwwroot/css/envelope-viewer.css

@@ -182,6 +182,8 @@ body.resizing {
     height: 100%;
     object-fit: contain;
     object-position: center;
+    image-rendering: -webkit-optimize-contrast;
+    image-rendering: crisp-edges;
 }
 
 .pdf-thumbnail__label {