fix: add packaged translation parity check

[rushijagani-dev]

Summary

• Add the missing packaged translation keys for ar-SA, da-DK, de-DE, it, ja, and ko.
• Add a backend regression test that checks every packaged locale JSON file against en-US.json for structural parity.

Fixes #2993

Testing

• python3 - <<PY ... PY direct packaged translation parity check: all 23 locale files match en-US structure
• python3 -m ruff check backend/tests/test_translations.py
• python3 -m ruff format --check backend/tests/test_translations.py
• git diff --check

Notes

• uv run pytest backend/tests/test_translations.py could not start locally because uv failed to parse the checked-in uv.lock at line 689 with missing field version.

---

Summary by cubic

Restore parity across packaged locale JSONs and add a regression test to enforce structure against en-US, fixing missing UI strings and preventing future drift. Fixes #2993.

• Bug Fixes
  • Filled missing keys: ar-SA/da-DK favorites.remove, ja fileUpload.browse, de-DE/it/ko DatePickerInput placeholders (single, range).
  • Added packaged translations parity test in backend/tests/test_translations.py to compare every locale file to en-US.json.

^{Written for commit b5f740c. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

_{Re-trigger cubic}