feat: add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• register Atlas Cloud as an optional built-in chat provider
• use the OpenAI-compatible API and authenticated model discovery endpoint
• add Atlas Cloud to the provider settings UI with English and Chinese labels
• keep all existing defaults and providers unchanged

Validation

• npm run typecheck:web
• npm run lint (0 errors; existing warnings only)
• Prettier check for all changed files
• locale JSON parsing
• git diff --check
• one live Atlas Cloud Chat Completions smoke request (no retry)

npm run typecheck and npm run build remain blocked by two pre-existing missing dependencies in untouched files: nanoid in ItemService.ts and the mdast type package in MarkdownLoader.ts.

[mrsibe]

Closing this as superseded by #44, not as a rejection of the feature — Atlas Cloud support is welcome.

#44 replaced the provider registry with protocol-based model connections and deleted three of the six files this PR touches:

• src/main/ipc/providerHandlers.ts — gone
• src/main/providers/registry/builtinProviders.ts — gone
• src/renderer/src/components/settings/ProvidersSettings.tsx — gone

Since Atlas Cloud is OpenAI-compatible, on the current main it no longer needs an implementation at all — it is a quick-setup preset. The whole change is a few lines in src/renderer/src/components/settings/ModelConnectionForm.tsx:

const QUICK_SETUPS: QuickSetup[] = [
  // ...
  {
    id: 'atlascloud',
    labelKey: 'presetAtlasCloud',
    protocol: 'openai-completions',
    baseUrl: 'https://api.atlascloud.ai/v1'
  }
]

plus a presetAtlasCloud label in src/renderer/src/locales/{en-US,zh-CN}/settings.json and a line in the README provider list. The protocol list itself is API_PROTOCOLS in src/shared/types/connection.ts, which also declares the embedding-capable protocols. Protocol implementations live in src/main/models/protocols/.

Two things that may help a fresh PR:

• npm run typecheck and npm run build now pass on main. The nanoid / mdast problems mentioned in the description were fixed by the npm packaging migration in build: fix packaged-app startup (npm migration + dependency boundary + smoke gate) #41.
• npm run check:design now enforces the UI conventions documented in DESIGN.md, so the settings UI part is even smaller than before.

If you'd like to reopen this as a fresh PR against the current model, I'd be glad to review it — the preset approach should be around a ten-line diff.