feat(frontend): add Chinese UI language toggle

[zxstar7789]

What does this PR do?

Adds a lightweight frontend internationalization layer with English and Simplified Chinese UI support.

This PR introduces:

• A small hardcoded i18n helper with en-US as the default and fallback locale.
• A persisted language setting in frontend settings.
• A language selector in Settings > Appearance.
• Simplified Chinese translations for the main frontend surfaces, including Sidebar, Chat, Settings, Dashboard, Logs, Get Started, Agents, and Data Sources entry points.
• No new runtime i18n dependency.

Connector setup tutorials and some backend/runtime strings remain in English to avoid changing third-party setup instructions or raw server-provided content.

How was this tested?

Tested locally on macOS with the frontend and backend running against Ollama.

Commands run:

cd frontend
npm run build
npm test

Manual testing performed:
Verified the default UI remains English.
Switched language to Simplified Chinese from Settings > Appearance.
Verified the main UI updates to Chinese.
Refreshed the browser and confirmed the language selection persists.
Switched back to English and confirmed the UI returns to English.
Verified the frontend loads at http://127.0.0.1:5173/.
Verified the backend health endpoint returns OK.
Verified model listing works with the local backend.

Checklist

• Tests pass (uv run pytest tests/ -v)
• Linter passes (uv run ruff check src/ tests/)
• Formatter passes (uv run ruff format --check src/ tests/)
• New/changed public API has docstrings
• Follows registry pattern (if adding new component)
• Documentation updated (if applicable)

Note: this PR only changes the frontend, so the Python uv run pytest, ruff check, and ruff format checklist items were not run. Frontend build and Vitest passed.

[karlberube]

Hello,
I speak French and I have submitted a request to obtain the translation files. Will I be able to set the language to French, since I speak Canadian French?
Thank you for your response.
Best regards,