Add throttling for loading indicator

[mkrause]

We already add a delay, but it would be good to also add a minimum time that the loading indicator is shown on screen, to reduce flickering.

See:

• https://blog.elijahlopez.ca/posts/react-how-to-add-minimum-loading-delay/
• https://mantine.dev/hooks/use-throttled-value/

Note: we could use this same approach for exit animations, for example the useDelayedUnmount in ModalProvider could probably be simplified with something like useThrottled(active, cond ? unmountDelay : 0).