feat: add Next.js application error boundary

[saurabhhhcodes]

Closes #987

Summary

• add a route-level Next.js error boundary for unexpected client crashes
• log the captured error for diagnostics without exposing details in the UI
• provide retry and return-to-home recovery actions

Validation

• git diff --check
• npx eslint app/error.tsx
• npx tsc --noEmit

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