feat(i18n): add Turkish login locale

[KeuqS]

Summary

• Add Turkish (tr) translations for the code-server login page.
• Register Turkish in the i18next resource bundle so it works with the existing locale selection and English fallback.
• Add a unit test that verifies the Turkish resource is available.

Testing

• jest --runInBand --forceExit --detectOpenHandles --collectCoverage=false test/unit/node/i18n.test.ts
• Result: 11 tests passed.
• prettier --check passed for all changed files.

[code-asher]

Thank you!