Design: a repository's following panel uses the design system, and its following-* classes leave §5d #3800

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. A fork's following panel is how the fork tree says how each fork follows.

Done:

• [ ] The panel that says how a fork follows its upstream is drawn from §5's components (src/web/ui.ts helpers, §5 classes) in place of .following .following-actions .following-eyebrow .following-foot .following-head .following-sentence, which src/web/style/content.ts defines today.
• [ ] Those classes' rules are gone, and their names leave the style/content.ts, page furniture 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/pages/following.ts, src/web/style/content.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.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3820 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Split into #4152 (the markup) and #4153 (the stylesheet and docs), so the design check stays green between them.