Load card images in background

[Fippe94]

It's annoying to have to mouseover a card and have it be blank for a little while before art is loaded. Load the art in the background (possibly this being optional, but I mean lofi art is already a thing if you need that), so that most art is already loaded when mousing over. Or even load all the art as the game starts, that is what pretty much every other game does.

[serprex]

Way to do this properly involves having a build step to put art into a spritesheet & use that. Then gets into whether to have both individual art alongside spritesheet to make it an option which method one uses. Kind of want to benchmark a bit, but I'd want to be using http2 before going by any results. May also be best to bucket the art by element

[serprex]

This is less of an issue due to a few changes made in past half decade

1. png converted to webp, making image size smaller
2. rust server rewrite has lower latency
3. cards in hand render card image so hovercard in match tends to have image loaded