fix(admin): keep image previews at natural aspect ratio

Media/gallery previews no longer stretch or crop: use object-fit contain,
natural sizing for single media, and the uncropped medium size instead of
the square thumbnail for gallery and native meta-box previews.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/js/admin-post-extension.js

@@ -34,7 +34,7 @@
       wp.media.attachment(id).fetch().then(function () {
         var data = wp.media.attachment(id).toJSON();
         var img = document.createElement('img');
-        img.src = (data.sizes && data.sizes.thumbnail ? data.sizes.thumbnail.url : data.url) || '';
+        img.src = (data.sizes && data.sizes.medium ? data.sizes.medium.url : data.url) || '';
         img.alt = data.alt || '';
         preview.appendChild(img);
       });