refactor: migrate pairs list to useApi

[webmirroring]

Summary

• Migrates the pairs list page from inline useEffect / useState fetching to the shared useApi<{ pairs: Pair[] }> hook.
• Preserves the existing loading, empty, populated, error, aria-live, aria-atomic, and aria-busy UI behavior.
• Documents the data-page convention for using useApi.
• Reconnects the quote page submit handler to the existing quote validators so the frontend build remains green.

Closes #154.

Accessibility notes

The pairs page keeps the same single polite live region, aria-atomic="true", loading aria-busy, empty-state copy, and role="alert" error behavior.

Validation

• node node_modules/jest/bin/jest.js src/app/pairs/page.test.tsx src/lib/__tests__/useApi.test.ts src/app/quote/page.test.tsx --runInBand passes
• node node_modules/jest/bin/jest.js --runInBand passes: 22 suites, 114 tests
• node node_modules/typescript/bin/tsc --noEmit passes
• node node_modules/next/dist/bin/next lint passes
• node node_modules/next/dist/bin/next build passes