Fix `wa-card` between border and content at corners

[lindsaym-fa]

Some of our new themes expose a gap between the border and content (especially images and headers and footers with background colors) of wa-card. The issue affects all themes; some are more noticeable than others.

[LeaVerou]

On it. I know what causes this. We set border-radius to inherit on .image, and since it is inherited, it does not account for subtracting the border width. Unless I come up with a better idea, it looks like we may need to reintroduce --border-radius and --border-width for this one. 😞

[LeaVerou]

Actually, we'd only need to reintroduce --border-width. In fact, it looks like it was removed by accident in eca15dc, since the CSS still uses it in calculations.

Alternatively, the CSS already has overflow: hidden, so we don't actually need .image to be rounded at all, simply not applying rounding to it would do it. This means the CSS also becomes more flexible, and can slot things that don't necessarily occupy the full width (e.g. icons like our overviews). However, keeping overflow: hidden there means we cannot have decorations that "escape" the card, which seems generally useful.

I'm slightly leaning towards simply reintroducing --border-width and removing overflow: hidden but I'm torn. @lindsaym-fa would love your thoughts on this tradeoff.

[lindsaym-fa]

I'd prefer reintroducing --border-width and removing overflow: hidden. Badges are a common case where content should be allowed to overflow a card, like this FA example: