Improve PDF thumbnail scaling and centering

Updated the `.pdf-thumbnail__canvas` CSS class to replace
`max-width`, `max-height`, and `display: block` with `width`,
`height`, `object-fit: contain`, and `object-position: center`.
These changes ensure the canvas fully occupies its container,
scales proportionally without cropping, and centers the content
for better visual consistency and usability.

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

@@ -178,9 +178,10 @@ body.resizing {
 }
 
 .pdf-thumbnail__canvas {
-    max-width: 100%;
-    max-height: 100%;
-    display: block;
+    width: 100%;
+    height: 100%;
+    object-fit: contain;
+    object-position: center;
 }
 
 .pdf-thumbnail__label {