fix(footer): make the footer the bottom gutter like in gex

[daggerok]

What

The footer is now the bottom gutter itself, the same way gex does it (FOOTER_CLASS in gex AttributionFooter.tsx): a box exactly as tall as the side padding (h-4, xl:h-8 where the layout uses xl:px-8), text centered in it; the content above has no bottom padding. The last card ends exactly one gutter above the viewport bottom, same as its left and right gap, whatever the footer text is

The previous version added the footer below a full gutter, so the bottom gap was about 42px against 16px on the sides

Checks

• Headless Chromium at 390, 1024, 1440 and 2000 px: card left gap = right gap = bottom gap (16px, 32px from xl where applicable), footer box flush with the page end
• bun run build