fix: update exam route parameter format and improve question index calculation

frontend/src/App.tsx

@@ -412,7 +412,7 @@ function AppInner() {
           }
         />
         <Route
-          path="/exam/:slug/q:n"
+          path="/exam/:slug/:n"
           element={
             <ExamRoute
               exam={exam}

frontend/src/components/ExamRoute.tsx

@@ -19,7 +19,7 @@ export default function ExamRoute({ exam, examState, onActivate, setExamState, o
   const { slug, n } = useParams<{ slug: string; n: string }>()
   const navigate = useNavigate()
 
-  const questionIdx = Math.max(0, (Number(n) || 1) - 1)
+  const questionIdx = Math.max(0, (Number((n ?? '').replace(/^q/, '')) || 1) - 1)
 
   // Deep link: restore from localStorage if no exam is active on mount
   useEffect(() => {