The home feed's cards draw §5's card, row and btn, and the .fcard-* classes leave feed.ts's markup #4267

For the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. Part one of two: this part changes only the markup; the stylesheet and docs/DESIGN.md are part two.

Done:

• [ ] A home feed card's who, why, reason, text, reply and form are drawn with the §5 component that fits (row, badge, bar, card, btn or field from src/web/ui.ts and §5's classes; name which in the pull). None of any .fcard-* class appears in the page's HTML. The reply and decide forms post the same fields and keep their same-origin check.
• [ ] Do not delete the rules from the stylesheet and do not edit docs/DESIGN.md. Part two does that, and the design check stays green because the classes are still defined and named.
• [ ] Nothing a person reads or taps is lost, long unbroken text still wraps (.break), and every form posts the same fields. A test asserts the old names are absent and the words, links and forms are present.

Files: src/web/studio/feed.ts, a test. Before pushing, run bun x tsc --noEmit, bun test test/design-lint.test.ts, bun scripts/design-lint.ts and the page's tests. No screenshots: the sandbox has no browser.