feat: implement leaderboard page and integrate into application routing

src/App.jsx

@@ -9,9 +9,7 @@ import Dashboard from './pages/Dashboard'
 import Admin from './pages/Admin'
 import Leren from './pages/Leren'
 import Testen from './pages/Testen'
-
-// Placeholder components for routing structure
-const Leaderboard = () => <div className="p-4 md:p-8"><h1 className="text-3xl md:text-4xl text-teal font-bold">Leaderboard</h1><p className="mt-4">See who is on top!</p></div>
+import Leaderboard from './pages/Leaderboard'
 
 // Protected Route Wrapper
 const ProtectedRoute = ({ children, requireAdmin }) => {