Align previous and next article navigation horizontally

[aaronpowell]

The Previous and Up next links are stacked vertically. Display them on the same row, with Previous left-aligned and Up next right-aligned. Preserve a readable stacked layout on smaller screens.

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[aaronpowell]

Fixed in 1a02fea.

.nextUp now uses a row layout with justify-content: space-between: Previous sits left-aligned, Up next sits right-aligned on the same row. Below 40rem it reverts to a stacked column layout (both links left-aligned) for small screens.

Verified on desktop (both links present), on the catalog's first item (only Up next present -- still right-aligns correctly via an explicit .nextUpNext modifier class rather than a positional :last-child selector), and at a 375px mobile viewport (stacked, left-aligned). Full npm run build passes (1097 pages).