a11y: announce new pair submission status

[vku2018]

Summary

• keep a polite role="status" node mounted in the new-pair form before submission
• announce Registering pair... while the create request is pending
• announce successful registration briefly before redirecting to /pairs
• clear the polite status on validation/backend errors so the existing role="alert" error path remains the only error announcement
• document the new-pair live-region convention in the README

Closes #153.

Validation

• PASS: npm test -- src/app/pairs/new/page.test.tsx --runInBand (10 tests)
• PASS: npm run lint
• PASS: git diff --cached --check
• BASELINE FAIL: npm run build fails on src/app/quote/Client.tsx:41 with Cannot find name 'assetsDiffer'
• BASELINE FAIL: npm test -- --runInBand fails only in src/app/quote/page.test.tsx with ReferenceError: assetsDiffer is not defined; 21 suites and 106 tests pass