Add translation key consistency check command and CI workflow

Add an automated translation consistency checker and run it in CI for pull requests.

Proposed scope:

1. Add a new Artisan command to compare translation keys against English.
2. Validate both JSON translations and nested PHP translation files.
3. Report missing keys per locale.
4. Report extra keys per locale (warning by default).
5. Return non-zero exit code when missing keys are found.
6. Optionally support failing on extra keys (for stricter mode).
7. Add unit tests for matching, missing, and extra key scenarios.
8. Run this check in a dedicated GitHub Actions workflow on pull requests.

I can prepare and submit a pull request for this feature if the maintainers agree with the proposed scope.