test(i18n): enforce CLDR plural completeness

[lilyshen0722]

Summary

• compare locale catalogs by normalized plural base keys instead of misleading raw path equality
• assert every pluralized key provides each CLDR category required by that locale
• add four English singular forms the new guard immediately exposed

English now has 929 raw paths and zh-CN has 925 by design: English requires _one and _other, while zh-CN requires only _other; normalized base-key parity remains exact. No Chinese copy changed.

Verification

• npm test -- --runInBand src/i18n/__tests__/translationKeys.test.ts
• npm test -- --runInBand src/i18n
• npm test -- --runInBand — 61 suites, 267 tests
• npx eslint --quiet src/i18n/__tests__/translationKeys.test.ts
• git diff --check

npm run typecheck remains blocked by three pre-existing errors in V2GithubPrCard.tsx and V2MessageBubble.tsx, outside this diff.