refactor(i18n): sync non-English locales + add CI enforcement for key parity

[prateekshourya29]

Description

Phase 2 of the i18n refactor. Closes the English-to-locale sync gap inherited from refactor-translations and adds a CI workflow that blocks future drift. No changes to the public API of @plane/i18n and no changes to how the app loads translations at runtime.

Locale sync: 100% key parity with English

• Every non-English locale was at partial coverage prior to this PR (≈87% on most, near-100% on de) with stale keys still hanging around from the old monolithic translations.ts.
• After this PR: all 18 non-English locales are at 3,837 / 3,837 keys — 100% parity with English.
• Each (locale, namespace) was rebuilt to match the canonical English key shape, filling each leaf with the appropriate per-locale translation. Stale keys were dropped.
• Translations are machine-quality and should be reviewed by native speakers before release. Tracked as follow-up work; not blocking this PR.

CI enforcement: .github/workflows/i18n-sync-check.yml

• New GitHub Actions workflow runs tsx packages/i18n/scripts/sync-check.ts --ci on PRs that touch packages/i18n/** or the workflow itself. Also runs on push to preview as a backstop.
• Fails any PR that introduces missing or stale keys.

Script refactor: shared locale I/O + better error context

• Extracted inline flattenKeys / loadLocale / LOCALES_DIR out of scripts/sync-check.ts into a new scripts/lib/locale-io.ts module.
• Added readJsonFile() helper that wraps JSON.parse errors with the offending file path — closes a CI-debugging gap where malformed locale JSON threw with no filename in the error.
• Behavior-preserving: check:sync output format, per-locale counts, and exit codes are identical to before.

Build tooling: pin tsx in pnpm catalog

• Replaced npx tsx@4.19.2 … invocations in @plane/i18n scripts with bare tsx …. npx tsx currently resolves to a broken release (tsx@4.21.0 declares an unpublished esbuild~0.27.0), causing local builds to fail with npm error code ETARGET.
• Pinned tsx: 4.20.6 in the workspace catalog and declared it as a devDependency of @plane/i18n so resolution goes through pnpm, not npx.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Improvement (change that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation update

Screenshots and Media (if applicable)

N/A — no visual changes. All existing translations continue to render identically; previously-missing keys now render their translated values instead of falling back to English.

Test Scenarios

• pnpm --filter=@plane/i18n run check:sync exits 0 with "All locales are in sync with English. No issues found."
• pnpm --filter=@plane/i18n run build succeeds; keys.generated.ts regenerates cleanly (3,837 keys × 28 namespaces).
• pnpm --filter=@plane/i18n check:lint / check:types / check:format all pass.
• actionlint .github/workflows/i18n-sync-check.yml passes.
• Switching to a non-English locale in user settings — verify previously-missing keys (e.g., empty-state titles, settings sidebar headers, command palette actions) render translated text rather than English fallback.
• Drift detection — introducing a missing or stale key on a test branch should cause the workflow to fail with a useful log pointing to the offending key.

References

• Builds on: refactor(i18n): migrate packages/i18n from MobX to react-i18next #8898 (the i18n MobX → react-i18next refactor that this PR completes the locale-side of)

Follow-ups (not in this PR)

• Native-speaker review of machine translations.
• Pluralization audit (ICU plural forms differ by language — Slavic languages have 4, Arabic 6).
• Translation memory / glossary for consistency of domain terms across locales.

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