Add mobile download link banner

[AvdLee]

Summary

• Add a mobile-only floating banner for visitors who cannot install the Mac app from their phone.
• Expand the banner into the Kit mobile download link form only when tapped, using form UID bd3a42d000.
• Track banner views, opens, dismissals, and mobile form submissions with Plausible events.

Test plan

• npm run lint
• npm run format:check
• npm run typecheck
• npm run build
• npm run knip