Add Atlas Cloud API provider preset

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as a built-in OpenAI-compatible API provider preset
• wire ATLASCLOUD_API_KEY and ATLAS_CLOUD_API_KEY environment fallback for provider credentials
• seed Atlas Cloud chat models for the existing canvas LLM path while leaving image/video model lists empty to avoid routing media models through incompatible OpenAI image/video endpoints
• add focused provider tests covering default registration, existing config merge behavior, env aliases, and chat provider resolution

Validation

• python3 -m pytest tests/test_atlascloud_provider.py -q
• python3 -m pytest tests/test_canvas_log_cleanup.py tests/test_atlascloud_provider.py -q
• PYTHONPYCACHEPREFIX=/tmp/infinite_canvas_pycache_20260724 python3 -m py_compile main.py tests/test_atlascloud_provider.py
• git diff --check
• Atlas Cloud live model catalog returned 374 visible models / 436 total models and confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

No README/docs/logo changes.