fix(stability): add Error Boundary to prevent full-page crashes

[gzhang33]

Summary

• Add React import to lib/utils/errorHandler.js (fixes broken createErrorBoundary)
• Wrap component tree with AppErrorBoundary in pages/_app.js

Problem

Multiple open issues report client-side crashes that crash the entire page (#2295, #3545, #3372). The root cause is third-party scripts (Mermaid, Live2D, Prism) mutating React-managed DOM. Adding an Error Boundary catches these errors and shows a fallback instead of a blank page.

Changed files

• lib/utils/errorHandler.js — add missing React import
• pages/_app.js — add AppErrorBoundary wrapping the app tree

Risks

• Low: Error Boundary is purely additive — only activates on uncaught rendering errors
• The fallback UI is minimal: error message + refresh button, inline styles only

Verification

yarn lint
yarn build
# To test: throw an error inside any component — should see fallback instead of blank page

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