Add CI check for translation coverage in pull requests

[Irozuku]

Summary

Adds a new GitHub Actions workflow to automatically validate translation coverage for both the frontend and backend on every pull request. This helps ensure that all supported locales remain fully translated and prevents incomplete translations from being merged.

---

Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

---

Changes (by file)

• .github/workflows/translations.yml: Added a new workflow that runs translation coverage checks for both frontend and backend during pull requests.
• .github/workflows/translations.yml: Added a frontend job that installs dependencies and runs i18next-cli status, failing if any locale does not have 100% translation coverage.
• .github/workflows/translations.yml: Added a backend job that sets up Python and runs scripts/check_translations.py lint to validate translation completeness.

---

Testing (optional)

• Open a pull request and verify that the workflow runs successfully when all translations are complete.
• Confirm that the workflow fails when a frontend or backend translation is missing.