Design: a spawned forge's next-steps card uses the design system, and the next-step* classes leave §5d #3802

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. It is the first thing a person sees in a forge sent to them by link.

Done:

• [ ] The first-login card of a spawned forge (core/next-steps.ts) is drawn from §5's components (src/web/ui.ts helpers, §5 classes) in place of .next-crew .next-step .next-step-apps .next-step-main .next-step-mark .next-step-title .next-steps, 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/next-steps.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.