test: add unit tests for window.onerror handler not re-logging errors caught by React error boundary

[samjay8]

Closes #680

Summary

• Adds a global error handler utility that prevents window.onerror from re-logging errors already caught by React error boundaries
• Modifies SectionErrorBoundary to mark caught errors so the global handler can skip them
• Provides a structured logging format with full error metadata for genuine unhandled errors
• Initializes the handler at app startup in main.tsx

Testing

• Unit tests cover boundary-caught errors, unhandled errors, duplicate prevention, and coexistence of both handlers
• All 14 new tests pass (src/utils/tests/globalErrorHandler.utils.test.ts)
• All 4 existing SectionErrorBoundary tests continue to pass

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