Show one question per card in the take-quiz screen, with prev/next arrows

Replaces the single long scrollable list of every question with one
card at a time plus arrow navigation, so a student's focus stays on
the current question instead of the whole quiz at once.

src/Client/Features/Quizzes/TakeQuiz/State.fs

@@ -53,6 +53,9 @@ let rec update (token: string option) (msg: Msg) (model: Model) : Model * Cmd<Ms
         updated, cmd
     | AnswerSaved _ -> { model with Error = None }, Cmd.none
     | AnswerSaveFailed(_, err) -> { model with Error = Some err }, Cmd.none
+    | GoToQuestion index ->
+        let clamped = index |> max 0 |> min (List.length model.Data.Questions - 1)
+        { model with CurrentIndex = clamped }, Cmd.none
     | Tick ->
         let updated = { model with RemainingSeconds = remainingSeconds model.Data DateTimeOffset.UtcNow }
         // The server is the actual authority (it now rejects any answer