Design: the boards' quick filters use the design system, and qf leaves §5d #3849

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. Boards are how the owner explores the forge's data.

Done:

• [ ] The quick filters on /boards and a board's view is drawn from §5's components (src/web/ui.ts helpers such as row, card, badge, btn, and §5 classes) in place of .qf, which src/web/style/content.ts defines today. src/web/pages/boards-view.ts also uses .qf: move it too.
• [ ] 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/pages/boards.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 failed and the decider saw no point in another try (pull #3902 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed and the decider saw no point in another try (pull #3904 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed and the decider saw no point in another try (pull #3902 closed)
2. the check failed and the decider saw no point in another try (pull #3904 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #3906 (the forms in boards.ts), #3907 (the forms in boards-view.ts) and #3908 (removing the .qf rules and the §5d entry, after both). Both pulls failed the type check on the same commit: boards.ts used the Html type without importing it (TS2304). The new issues say to import it, to update the qf regex in test/boards-selectors.test.ts, and they drop the screenshot step, which a builder without a browser cannot do.