feat: add Atlas Cloud LLM provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an optional OpenAI-compatible LLM component
• expose its API key through the masked settings flow and Docker environment
• document setup in English and Chinese while preserving the existing default provider
• cover config discovery, client initialization, display naming, and secret masking

Validation

• uv run --python 3.12 --frozen pytest tests/unit/test_config.py tests/unit/test_openai_plugins.py -q (32 passed)
• Ruff checks passed for the changed Python tests
• frontend production build passed
• Go files formatted with gofmt
• go test ./internal/api was attempted but this environment lacks the native pkg-config/Opus dependency required by the existing LiveKit package
• one live streaming Atlas Cloud Chat Completions smoke request returned HTTP 200 and a completed SSE stream; no retry
• git diff --check passed