#3636070: Show that the preview is loading, rather than an empty white frame

Closes #3636070.

The preview pane showed a blank white frame until its page loaded, which on a cold site, or the first front-end page after a cache rebuild, could take many seconds. A page the frame doesn't show yet is now hidden behind the pane's background and Drupal's throbber, "Loading the page…", until it loads. A reload of the same page keeps it in view, as before, so the scroll position holds.

I used an LLM in the production of this code.

ChatPreviewTest checks that the loading state clears; Selenium ran it on Drupal 11 from this Git branch.