Add React Error Boundary

[satsdisco]

Summary

Currently there is no error boundary — any unhandled React error crashes the entire app with a white screen. For a publishing tool where people draft long-form content, this is unacceptable. A crash during writing could mean lost work.

Implementation

• Add a top-level ErrorBoundary component wrapping the app
• Show a friendly "Something went wrong" screen with:
  • Error message (dev only)
  • "Reload" button
  • "Your draft has been auto-saved" reassurance (if draft exists in localStorage)
• Add route-level error boundaries for isolated failures (reader crash shouldn't kill editor)
• Consider react-error-boundary package or a simple class component

Acceptance Criteria

• App never shows white screen on error
• Draft content is never lost due to a crash
• Errors are logged to console for debugging
• Route-level boundaries isolate failures