Improve quiz page layouts: WIDM-style answers and responsive centering

- Add green square answer buttons styled after the TV show
- Two-column answer grid for 6+ answers, single column on mobile
- fit-content centering for question pages so block matches question width
- Narrow fixed-width centering for form pages (enter name, select season)

templates/quiz/select_season.html.twig

@@ -1,5 +1,7 @@
 {% extends 'quiz/base.html.twig' %}
 {% block body %}
-    {{ form(form) }}
-    <a href="{{ path('tvdt_backoffice_index') }}" class="btn btn-outline-secondary">{{ 'Manage Quiz'|trans }}</a>
+    <div class="quiz-form-narrow">
+        {{ form(form) }}
+        <a href="{{ path('tvdt_backoffice_index') }}" class="btn btn-outline-secondary">{{ 'Manage Quiz'|trans }}</a>
+    </div>
 {% endblock body %}