Design: the front page's hero and its terminal line use the design system, and hero/hero-terminal leave §5d #3870

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. The front door is the first thing a visitor sees; its hero should be built from the same parts as the rest.

Done:

• [ ] The front page's hero and its terminal line is drawn from §5's components (src/web/ui.ts helpers such as row, card, badge, btn, and §5 classes) in place of .hero .hero-terminal, which src/web/style/content.ts defines today. The terminal line still prints FORGE_APP_SSH verbatim when it is set, and says the terminal is off when it is not.
• [ ] Those classes' rules are gone, and their names leave their 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, and every form posts the same fields.
• [ ] The page's existing tests pass. A test asserts the old class names are absent from the page's HTML.

Files: src/web/studio/home.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 #3891 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Split into #4170 (the markup) and #4171 (the stylesheet and docs), so the design check stays green between them.