fix(PictureThumbnail): Scope image CSS to direct children

The [loading] opacity rule used a descendant selector which also matched
the cloned img that CropperJS moves into its own canvas inside the
thumbnail. If the clone's load event fired before the original's, the
cropper UI was built while the loading attribute was still set, leaving
the image inside the cropper invisible until the original fired its own
load event. Scoping the rule to direct children keeps the hide/reveal
behavior for our own placeholder while leaving nested imgs untouched.

Author: tvdeyen

app/assets/builds/alchemy/admin.css

@@ -1,11 +1,11 @@
 alchemy-picture-thumbnail {
   &[loading] {
-    img {
+    > img {
       opacity: 0;
     }
   }
 
-  img {
+  > img {
     opacity: 1;
     transition: opacity var(--transition-duration);
   }