feat(i18n): add Turkish (tr-TR) language support

[esershnr]

Description

This PR adds full Turkish (tr-TR) language support to Artalk, covering the backend, frontend, sidebar, and configuration templates.

Proposed Changes:

• Backend: Added i18n/tr.yml for core translations.
• Frontend: Added ui/artalk/src/i18n/tr.ts (using defineLocaleExternal for performance and tr/tr-TR compatibility).
• Sidebar/Admin: Added ui/artalk-sidebar/src/i18n/tr.ts.
• Config Templates:
  • Updated conf/artalk.example.yml to include tr locale.
  • Created conf/artalk.example.tr.yml with translated setting descriptions for the admin panel.

Verification:

• CONTRIBUTING.md Compliance: Followed all prescribed development workflows.
• Automated Tests: Ran pnpm test in ui/artalk - 33 passed, 0 failed.
• Manual/Visual Verification:
  • Built a fresh Docker image (artalk-turkish-test) to ensure perfect integration.
  • Verified user interface elements (nick, email, send buttons, placeholders) and admin panel descriptions visually.
• Internal Consistency: Ran go generate ./internal/config to synchronize configuration schemas.