Design: the board (/board) is built from the design system, and its board-* classes leave §5d #3165

Part of #2678. src/web/pages/board.ts, the live wall of the crews at work, carries the largest page-local set in §5d (.board-*).

Done:

• [ ] The board uses §5 components and tokens; no .board-* class remains, or the few that are truly a new component move into the §5 table with a one-line reason.
• [ ] At 360px it reads as one column with no horizontal scroll.

Design rule (#2678): one page, nothing else. Done also means the pull's looks show this page before and after, desktop and 360px, light and dark (if the looks run does not record dark separately yet, see #3126, the pull description links the four after-pictures from its look run). Every class it stops using leaves docs/DESIGN.md §5d in the same pull, and bun scripts/design-lint.ts passes. Risk: the class rule fails a §5d name whose last rule is gone, so remove the rule and the name together.