test: cover new-pair submit states

[es3298]

Summary

• update the new-pair page tests to mock apiPost directly through the app import path
• add coverage for the in-flight Saving... submit state while the pair registration request is pending
• document the covered new-pair form flows in the README testing notes
• add a Jest @/ module mapper so tests can mock app aliases consistently

Closes #142

Validation

• npm test -- src/app/pairs/new/page.test.tsx --runInBand - 9 tests passed
• npm run lint - passed
• git diff --check - passed
• npm test -- --runInBand - blocked by existing upstream failures in src/app/quote/Client.tsx (assetsDiffer is referenced but not defined) and src/lib/__tests__/theme.test.ts
• npm run build - blocked by existing upstream src/app/quote/Client.tsx assetsDiffer type error