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/css/admin/post-extension.css

@@ -88,7 +88,8 @@
   width: 100%;
   aspect-ratio: 1;
   border-radius: 3px;
-  object-fit: cover;
+  object-fit: contain;
+  background: #f6f7f7;
 }
 
 .aksrt-post-extension__radios {