test: cover new pair form flow

[gaoflow]

Summary

• add test coverage for the /pairs/new form contract
• mock useRouter and apiPost to verify redirect behavior without navigation
• cover identical assets, successful submit, server error, and in-flight loading state

Closes #17

Tests

• npm test -- src/app/pairs/new/page.test.tsx --runInBand
• npm run lint
• npm test -- --runInBand
• npm test -- --coverage --runInBand
• npm run build

Coverage note: app/pairs/new/page.tsx reports 100% statement, branch, function, and line coverage in the coverage run.