Improve quiz layout: add fixed topbar, include navigation, and clean up unused elements

- Add `.quiz-topbar` with fixed positioning and spacing in `quiz.scss`
- Update `base.html.twig` to include `quiz/nav.html.twig` in a new `nav` block
- Remove unused "Manage Quiz" button from `select_season.html.twig`

assets/styles/quiz.scss

@@ -85,6 +85,15 @@ input.btn-check:checked + label.answer-btn {
     }
 }
 
+.quiz-topbar {
+    position: fixed;
+    top: 1rem;
+    right: 1rem;
+    z-index: 1030;
+    display: flex;
+    gap: 0.5rem;
+}
+
 .elimination-screen {
     position: fixed;
     top: 0;