docs: add UI design-tokens styleguide

[designartvis]

Closes #152

Summary

• add docs/STYLEGUIDE.md documenting Badge variants, Button variants, focus-ring conventions, theme colors, Tailwind scope, and reduced-motion behavior
• link the contributor styleguide from README.md
• restore the quote form's missing validation imports/normalized values so the existing test and build suites pass

Validation

• npm run lint
• npm test -- --runInBand - 22 suites / 114 tests passing
• npm run build
• git diff --check -- README.md docs/STYLEGUIDE.md src/app/quote/Client.tsx

Notes

This started as a documentation-only issue, but the repository's current main could not pass npm test or npm run build because src/app/quote/Client.tsx referenced assetsDiffer, isValidAmount, and normalized request values without defining them. The code change is intentionally minimal and keeps the existing checks green.