frontend: Configure aspect-ratio directly for .preview

frontend/src/main.less

@@ -71,10 +71,9 @@ main {
 }
 
 .preview {
-	/* TODO: this should all be slightly more dynamic, more the size of a real A4 paper, and the content scaled appropriately */
 	width: 95vw;
-	/* width/√2, aka A4 aspect ratio */
-	height: 67vw;
+	/* A4 aspect ratio: √2:1 */
+	aspect-ratio: 1.4142135623730951;
 
 	background-color: #fff;
 	color: #000;