Settings › Passkeys speaks the home page's language #9726

For: the page where a person keeps their way in is the next page in the design pass that #9706–#9714 carry.

Done looks like:

• [x] Settings › Passkeys (src/web/pages/passkeys.ts) follows the opening principles of docs/DESIGN.md: it uses space where a border or box does no work, and the passkey rows and the add action use the home page's type.
• [x] Nothing is lost: listing, adding and removing still work; removing the last way in is still refused with the words in test/passkeys-last-way-in.test.ts; every change is still checked for same origin.
• [x] The page fits a 390 px phone with no sideways scroll.
• [x] bun scripts/design-lint.ts passes, and so do test/passkey*.test.ts and test/passkeys*.test.ts.

Files: src/web/pages/passkeys.ts, plus its area doc under docs/design/areas/ if a class is added. Risks: the add ceremony runs a small script, because navigator.credentials needs one. Keep it working, and keep the page readable and the remove form working with JavaScript off.

Reviewed pull #9736 at its head: ticked Settings › Passkeys (src/web/pages/passkeys.ts) follows the opening principles of docs/DESIGN.md: it uses space where a border or box does no work, and the passkey rows and the add action use the home page's type.; Nothing is lost: listing, adding and removing still work; removing the last way in is still refused with the words in test/passkeys-last-way-in.test.ts; every change is still checked for same origin.; The page fits a 390 px phone with no sideways scroll.; bun scripts/design-lint.ts passes, and so do test/passkey*.test.ts and test/passkeys*.test.ts..