Design: a home feed card is the design system's card, and the fcard-head/acts/more classes leave §5d #3166

Part of #2678. src/web/studio/feed_style.ts defines the feed's own .fcard* card.

Done:

• [ ] A feed item renders with card() (head, body, actions) from the design system; .fcard, .fcard-head, .fcard-acts, .fcard-more and their rules are gone. The composer and the apps rail are out of scope.
• [ ] Home's budget test (#2675) still passes; no layout shift (the card's height does not change after the live fragment swaps in).

Design rule (#2678): one page, nothing else. Done also means the pull's looks show this page before and after, desktop and 360px, light and dark (if the looks run does not record dark separately yet, see #3126, the pull description links the four after-pictures from its look run). Every class it stops using leaves docs/DESIGN.md §5d in the same pull, and bun scripts/design-lint.ts passes. Risk: the class rule fails a §5d name whose last rule is gone, so remove the rule and the name together.