Row subgrid, and a little bird in the update card

Two things. CSS subgrid now works on the row axis: a grid item that opts into its parent's rows lines its own content up to the parent's row lines, so layouts where every card's heading and body must line up across a row render the way they do in the reference browser (column subgrid already worked). And the software-update card gains a bit of character: while an update downloads, a small Floati bird flies through a scrolling course of gates toward the Floati emblem, a different random run each time, speeding up as the download fills and landing home when it finishes. It honours reduced-motion. Previous release recap: update checks now run the moment you open the app and once a day after, with a page showing update activity.