feat(i18n): add CI translation drift check (ENG-243)

[patoo0x]

Summary

Add CI enforcement for translation drift by failing when any language is missing keys from app/i18n/raw-i18n/source/en.json.

Changes

• Added workflow: .github/workflows/i18n-drift.yml
  • Runs on PRs that touch app/i18n/**
  • Runs yarn check:translation-drift
• Added script: utils/check-translation-drift.js
  • Flattens nested keys
  • Reports missing keys per language
  • Exits non-zero on drift
• Added package script: check:translation-drift
• Added unit tests for key comparison logic: utils/check-translation-drift.test.mjs

Notes

• This intentionally blocks i18n PRs with missing keys (the exact regression ENG-243 is meant to prevent).
• ENG-241/ENG-242 should merge first to clear current drift baseline.

[linear]

ENG-243 i18n Phase 3: CI enforcement — fail build on translation drift