Add Atlas Cloud LLM preset

[binyangzhu000-sudo]

Summary

• add Atlas Cloud constants and API key resolution for ATLASCLOUD_API_KEY / ATLAS_CLOUD_API_KEY
• add an Atlas Cloud preset in the LLM configuration sidebar
• reuse the existing OpenAI-compatible ask_gpt path and model-list fetcher with the resolved Atlas Cloud key
• add focused tests for Atlas Cloud base URL detection and API key fallback behavior

Validation

• python3 -m pytest tests/test_atlascloud_config.py -q
• PYTHONPYCACHEPREFIX=/private/tmp/videolingo-pycache python3 -m py_compile core/utils/config_utils.py core/utils/ask_gpt.py core/st_utils/sidebar_setting.py tests/test_atlascloud_config.py
• git diff --cached --check
• Atlas Cloud live model catalog: 374 visible models; confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

Notes

• No README changes.
• No sponsor, logo, credits, or partner promotion changes.
• ruff was not run because it is not installed in this local environment.