Design: the front page's featured app uses §5's card and look, and front-featured* leave §5d #3940

For: the design audit (#2678). One page part at a time onto §5's components (src/web/ui.ts helpers such as row, card, badge, btn, field, empty, and §5 classes), so docs/DESIGN.md §5d only shrinks.

Done:

• [ ] The featured section src/web/studio/home.ts draws (.front-featured, .front-featured-look, .front-featured-body) is drawn from §5's card and the look image the app tiles use, with the same link, image and text.
• [ ] Those classes' rules are gone from src/web/style/content.ts (and from the reduced-motion block, if listed there), and their names leave the style/content.ts bullet of docs/DESIGN.md §5d. bun scripts/design-lint.ts passes, and --update gives a lower or equal baseline.
• [ ] Nothing a person reads or taps is lost, and every form posts the same fields. A test asserts the old class names are absent from the page's HTML, the page's existing tests pass, and bun x tsc --noEmit passes before the push. Screenshots are not required; the sandbox has no browser.

Files: src/web/studio/home.ts, src/web/style/content.ts, docs/DESIGN.md, the test. Risk: another page borrowing a class. git grep each name first, and if one is used elsewhere, keep it and say so in the pull. #3850 and #3870 also edit home.ts: rebase onto whichever lands first.

Parked by the crew: the check failed ("a pull whose review loop is spent is the planner's (#1655) > the planner is asked for it and its brief says three reviews asked for changes") and the decider saw no point in another try (pull #3954 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("design check: nothing added outside the system (#2678) > the tree is no worse than its baseline") and the decider saw no point in another try (pull #4028 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed ("a pull whose review loop is spent is the planner's (#1655) > the planner is asked for it and its brief says three reviews asked for changes") and the decider saw no point in another try (pull #3954 closed)
2. the check failed ("design check: nothing added outside the system (#2678) > the tree is no worse than its baseline") and the decider saw no point in another try (pull #4028 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #4084, which waits for #4063: the design check's baseline test parked it.