fix(build): merge React vendors into one chunk to fix white screen

[tcp404]

Description

Fixes a full white screen in every packaged / statically-served build (desktop dmg + electron-vite preview, the WebUI toggle, and aionui-web).

Root cause (build-level, on main). packages/desktop/electron.vite.config.ts manualChunks split React and its tightly-coupled vendors into separate chunks (vendor-react, vendor-arco, vendor-highlight, vendor-markdown, vendor-editor), producing circular ESM chunk imports:

vendor-editor -> vendor-highlight -> vendor-arco -> vendor-react -> vendor-editor
vendor-highlight -> vendor-markdown -> vendor-highlight

With a chunk cycle, ESM evaluation order left React's exports uninitialized when vendor-arco's top level ran React.createContext, throwing:

Uncaught TypeError: Cannot read properties of undefined (reading 'createContext')
  at vendor-arco-*.js:1

#root stayed empty → 100% white screen. Dev (bun start) is immune because Vite serves unbundled ESM (no manualChunks). All three production consumers serve the same out/renderer, so all three white-screen.

Fix. Co-locate react / react-dom / @arco-design / markdown / highlight / editor / katex into a single vendor chunk, removing the cross-chunk cycle. A single vendor chunk loads from disk (file://) so the extra granularity bought nothing but the cycle.

Type of Change

• fix — Bug fix (non-breaking change which fixes an issue)

Atomic PR Checklist (Rule 1)

• This PR contains exactly one bug fix
• The PR title follows Conventional Commit format

Local Checks (Rule 3)

• bun run format — passes (via just push)
• bun run lint — 0 errors (via just push)
• bunx tsc --noEmit — passes (via just push)
• bunx vitest run — 3964 passed / 5 skipped (via just push)
• i18n validated (via just push)

Runtime Verification

• Verified on macOS
• Verified on Windows
• Verified on Linux
• I have performed a self-review of my own code

Verified on macOS:

• bun run package → zero circular-chunk warnings (was 2); single merged vendor chunk.
• Production build (electron-vite preview, file://out/renderer) at 390px: #root non-empty, React mounts, no longer white (was rootLen=0 / 100% white / createContext crash).
• WebUI (bun run webui) at 390px: renders, React mounts.
• File preview at 390px in a project conversation: text / image / pdf / office all render.

Additional Context

Chunk-boundary / build-config only; no runtime/source behavior change. Unblocks the mobile WebUI file-preview goal (that path serves the same out/renderer).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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