[client] Add CI check for translation key parity (#6852)

English (en) is the source of truth for UI translation keys; the other
nine locales rely on runtime English fallback for any missing key, so a
gap never surfaces in CI. Add a dependency-free Node check that fails
when any locale declared in _index.json does not carry the exact same
key set as en (missing or orphaned keys), wired into a dedicated
UI Translations workflow that runs on locale changes.

Also close the one existing gap the check found: ja was missing
daemon.outdated.download ("Download Latest").

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/ui/frontend/package.json

@@ -15,7 +15,8 @@
     "lint": "eslint \"src/**/*.{ts,tsx}\"",
     "lint:fix": "eslint \"src/**/*.{ts,tsx}\" --fix",
     "check": "pnpm lint && pnpm typecheck && pnpm format:check",
-    "check:fix": "pnpm lint:fix && pnpm format && pnpm typecheck"
+    "check:fix": "pnpm lint:fix && pnpm format && pnpm typecheck",
+    "i18n:check": "node ../i18n/check-translations.mjs"
   },
   "dependencies": {
     "@radix-ui/react-dialog": "^1.1.15",