feat: add Atlas Cloud AI playground provider

[binyangzhu000-sudo]

Summary

• Add Atlas Cloud to the AI Playground provider list with the OpenAI-compatible base URL https://api.atlascloud.ai/v1.
• Route Atlas Cloud through the existing OpenAI-compatible model discovery, chat completion, and streaming response parsing paths.
• Pre-fill deepseek-ai/deepseek-v4-pro as the Atlas Cloud default model while preserving saved user provider settings.

Validation

• npm install --package-lock=false --ignore-scripts
• npx prettier --check site/src/components/AIPlayground/ConfigPanel.tsx site/src/components/AIPlayground/Service.ts site/src/components/AIPlayground/constants.ts site/src/components/AIPlayground/types.ts
• npm run build:esm
• npm --prefix site run tsc
• npm --prefix site run build:static
• git diff --check
• Atlas Cloud live model catalog check returned 437 models and confirmed deepseek-ai/deepseek-v4-pro and qwen/qwen3.5-flash.

Notes: the site build reports existing Next/MDX warnings about <img> usage, missing image alt attributes, no lockfile cache stability, and one large-page-data warning; the build exits successfully.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.