Plan: phase9/error-handling

Goal

Implement global error handling UX across backend and frontend for consistent error responses, user-facing notifications, and graceful failure recovery.

Changes

Backend (cmmc/app.py)

• RequestValidationError handler: Returns 422 with {detail, error_code: "VALIDATION_ERROR", errors} format
• Unhandled Exception handler: Returns 500 with {detail: "An internal server error occurred.", error_code: "INTERNAL_ERROR"}, logs full traceback

Frontend

• react-hot-toast: Installed as toast notification library
• ErrorBoundary (ui/src/components/ErrorBoundary.tsx): React error boundary catching component crashes, shows error card with Refresh/Try Again buttons
• NotFoundPage (ui/src/components/NotFoundPage.tsx): 404 page with navigation back to dashboard
• useApiErrorHandler (ui/src/hooks/useApiErrorHandler.ts): Hook for pages to handle API errors with toast notifications (skips 401s, handles network errors)
• App.tsx: Added Toaster provider, ErrorBoundary wrapper, catch-all * route for 404

Tests

• Backend: 5 tests for validation error format, 404 handling, health endpoint
• Frontend: 6 tests for ErrorBoundary and NotFoundPage components