Issue 003: Green v2 design system

[cody257]

Summary

• Replaced the purple/blue prototype theme with a restrained green v2 direction.
• Added shared UI primitives: BrandMark, Button, and Surface.
• Updated landing, buyer placeholder, seller workspace, form, response cards, copy button, and toast to use the new design language.
• Added lucide-react for consistent icons.

Verification

• npm run lint
• npm test
• npm run build
• npm audit --audit-level=moderate
• Desktop/mobile landing screenshots checked locally with Playwright CLI.

Notes

• This PR intentionally keeps product behavior the same. Mock chat/dashboard work should happen in later issues.