fix: avoid layout shifts when loading characters

The layout jumps a bit when the answer is revealed.

This is most noticable in the read hanzi cards, where the layout jumps a bit.

The layout shifts probably happen after the SVG data for the characters is loaded, at which point space gets reserved for them in the layout for the first time. With a little CSS tweaking, we can probably pre-allocate this space beforehand, so that no layout shifts happen.