Add invite links

frontend/src/App.tsx

@@ -14,6 +14,7 @@ import { MarathonPage } from '@/pages/MarathonPage'
 import { LobbyPage } from '@/pages/LobbyPage'
 import { PlayPage } from '@/pages/PlayPage'
 import { LeaderboardPage } from '@/pages/LeaderboardPage'
+import { InvitePage } from '@/pages/InvitePage'
 
 // Protected route wrapper
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
@@ -43,6 +44,9 @@ function App() {
       <Route path="/" element={<Layout />}>
         <Route index element={<HomePage />} />
 
+        {/* Public invite page */}
+        <Route path="invite/:code" element={<InvitePage />} />
+
         <Route
           path="login"
           element={