Configure React Router for proper page navigation

[ieliofficial]

react-router and react-router-dom are installed (see package.json) but not used. Currently navigation between landing page and problem details uses conditional rendering.

Requirements:

1. Set up Routes in App.jsx with:
   • / - Landing page
   • /problems/:id - Problem detail view
2. Use useParams in ProblemSolutionPage to get the problem ID
3. Update navigation to use and instead of state
4. Ensure browser back button works correctly