feat: Upgrade React from 18.2.0 to 19.0.0

[devin-ai-integration]

feat: Upgrade React from 18.2.0 to 19.0.0

Summary

This PR upgrades the invoice application from React 18.2.0 to React 19.0.0, implementing a systematic 4-phase migration approach. The upgrade includes React core packages, React DOM, testing libraries, and the lucide-react icon library to ensure full React 19 compatibility.

Key Changes:

• ✅ React Core: react and react-dom upgraded to ^19.0.0
• ✅ Testing Libraries: Updated @testing-library/jest-dom (5.16.5 → 6.0.0), @testing-library/react (13.4.0 → 16.0.0), and @testing-library/user-event (13.5.0 → 14.0.0)
• ✅ Icon Library: Updated lucide-react (0.220.0 → 0.544.0) for React 19 compatibility
• ✅ Backward Compatibility: All existing React patterns (hooks, context, routing, StrictMode) work without code changes

Manual Testing Completed:

• Authentication flow (login/registration) ✅
• Navigation between routes (dashboard, invoices, reports) ✅
• React hooks functionality (useNavigate, useLocation, useAuth) ✅
• Notification system with react-toastify ✅
• Backend API integration ✅

Review & Testing Checklist for Human

⚠️ HIGH PRIORITY - Please test thoroughly before merging (3 critical items):

• End-to-End Functionality Testing: Test complete user workflows (registration → login → create invoice → generate reports) to ensure no regressions
• Console Error Monitoring: Check browser console for any new React 19 related errors, warnings, or deprecation notices during normal usage
• Cross-Browser Compatibility: Test the application in different browsers (Chrome, Firefox, Safari) as React 19 may have different behavior across browsers

Notes

• React 19 is a major version upgrade that could introduce subtle behavioral changes despite backward compatibility promises
• No existing test suite exists for the frontend (npm test shows "No tests found"), so manual testing is critical
• Backend tests pass (7/7) confirming no API integration issues
• React Router shows future flag warnings which are informational only and don't affect functionality

Link to Devin run: https://app.devin.ai/sessions/ad2e706387f44ca19fe23609e082eb71
Requested by: @JRWu (jia.wu@codeium.com)

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