Sign-in's passkey lead and note draw §5's btn and muted text, and .passkey-lead, .passkey-note leave the markup #4271

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:

• [ ] Sign-in's and sign-up's passkey lead and its notes 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 .passkey-lead, .passkey-note appears in the page's HTML. Keep .passkey, .passkey-on, .passkey-off: they are the script's show/hide hooks, not styling. passkey-client.ts writes into the note: if it finds it by class, find it by a data- attribute instead. With no script the password form still signs in.
• [ ] 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/pages/site.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.

Parked by the crew: the check failed ("web pages end to end > the home carries no error-note for a setting that is not the default") and the decider saw no point in another try (pull #4278 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.