Add Atlas Cloud model provider preset

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as a built-in OpenAI-compatible model provider preset
• support ATLASCLOUD_API_KEY, optional ATLASCLOUD_API_BASE, ATLASCLOUD_MODEL, and ATLASCLOUD_MODEL_2 environment fallback when no database model config exists
• expose Atlas Cloud in the model settings provider picker and align provider type schemas used by IPC/tool handlers

Validation

• pnpm install --ignore-scripts
• pnpm exec tsc -p tsconfig.renderer.json --noEmit
• pnpm run build:renderer
• changed-file TypeScript transpile check
• git diff --check
• live Atlas model catalog check confirmed qwen/qwen3.5-flash and deepseek-ai/deepseek-v4-pro

Full pnpm run type-check / pnpm run build:main are currently blocked by the existing missing form-data dependency in src/main/tools/feishu-doc-rich-blocks.ts. Changed-file checks and renderer build pass.

No README/docs changes, no sponsor/logo/credits/partner additions.