Create and mount the centered "Loading scene..." indicator into the viewer
container. The indicator is a small box fixed to the viewport centre carrying
a spinner and a text label (it does not cover the viewport); it is given a
fixed id so hideLoadingIndicator can find and remove it later.
Returns HTMLElement
The mounted indicator element, so the caller can update its text or
remove it directly.
Create and mount the centered "Loading scene..." indicator into the viewer container. The indicator is a small box fixed to the viewport centre carrying a spinner and a text label (it does not cover the viewport); it is given a fixed id so hideLoadingIndicator can find and remove it later.