Design: the held board (boards-held) is built from the design system, and its held-* classes leave §5d #3199

Same shape as #3183 and #3188: the held board (boards-held) uses the design system's rows, cards, tables and tokens (docs/DESIGN.md §5), and its page-local .held-* classes leave the §5d register.

Done looks like:

• [ ] src/web/pages/boards-held.ts uses design-system classes; every .held-* rule is deleted from src/style/ and from §5d in docs/DESIGN.md.
• [ ] test/design-lint.test.ts and test/design-audit.test.ts pass, with the register smaller.
• [ ] The look of the page in light and dark has no layout shift and no lost information against main (the pull's before/after looks).

Files: src/web/pages/boards-held.ts, src/style/*.ts, docs/DESIGN.md. Risk: tests that match on the old class names need updating to the new ones. Say which ones in the pull.