Design: the studio's head, tell box and shelf rows use the design system, and their studio-* and shelf-* classes leave §5d #3616

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks.

Done:

• [ ] The studio's app head, tell box, word and shelf rows is drawn from §5's components (src/web/ui.ts helpers, §5 classes) in place of .fcard-app .shelf-desc .shelf-facts .shelf-meta .studio-ask-actions .studio-head .studio-tell .studio-word, which src/web/studio/style.ts defines today.
• [ ] Those classes' rules are gone and their names leave the src/web/studio/style.ts bullet of docs/DESIGN.md §5d. bun scripts/design-lint.ts passes, and so does --update with a lower or equal baseline.
• [ ] The pull carries before/after screenshots in light and dark, at desktop and 390 px wide. Nothing a person reads or taps is lost.
• [ ] The page's existing tests pass. A test asserts the old class names are absent from the page's HTML.

Files: src/web/studio/style.ts, docs/DESIGN.md, the page's test. Risk: another page borrowing one of these classes. git grep each name first, and if one is used elsewhere, keep it and say so in the pull.