fix(i18n): enforce bundled locale parity

[lntutor]

Fixes #2993

Restore the six missing bundled translation keys and add a regression test that compares every shipped locale against en-US.json. Future key drift now fails with the affected locale and key in the pytest diff.

Validation:

• uv run pytest backend/tests/test_translations.py (35 passed)
• uv run scripts/lint.py backend/tests/test_translations.py
• uv run scripts/format.py --check backend/tests/test_translations.py

---

Summary by cubic

Enforces full parity between all bundled locales and en-US.json by restoring six missing keys and adding a parity test. Fixes #2993.

• Bug Fixes
  • Restored favorites.remove in ar-SA and da-DK.
  • Restored fileUpload.browse in ja.
  • Added components.DatePickerInput.placeholder.{single,range} in de-DE, it, and ko.
  • Added a regression test in backend/tests/test_translations.py that compares every shipped locale to en-US.json and fails on drift.

^{Written for commit 74edef0. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

_{Re-trigger cubic}

[lntutor]

Closing to avoid competing with the earlier #2994 and #2995 translation-parity fixes. Sorry for the overlap.