feat: replace fetch-based modal forms with Turbo Frames (#181)

Enable @hotwired/turbo with Drive explicitly disabled, then migrate the
bo--modal-form Stimulus controller (custom fetch + X-Modal-Request pattern)
to a thin bo--modal controller that lets Turbo handle HTTP and DOM swap.
Adds frame templates for quiz questions and bank questions; controllers now
detect Turbo-Frame header instead of X-Modal-Request.

templates/backoffice/season/tab_tests.html.twig

@@ -2,9 +2,9 @@
     <div class="col-md-6 col-12">
         <div class="d-flex align-items-center gap-2 mb-3">
             <a class="btn btn-sm btn-outline-primary"
-               href="{{ path('tvdt_backoffice_quiz_add', {seasonCode: season.seasonCode}) }}">{{ 'Import Quiz from Excel'|trans }}</a>
-            <a class="btn btn-sm btn-outline-secondary"
                href="{{ path('tvdt_backoffice_quiz_add_blank', {seasonCode: season.seasonCode}) }}">{{ 'Add Empty Quiz'|trans }}</a>
+            <a class="btn btn-sm btn-outline-secondary"
+               href="{{ path('tvdt_backoffice_quiz_add', {seasonCode: season.seasonCode}) }}">{{ 'Import Quiz from Excel'|trans }}</a>
         </div>
         <div class="list-group mb-3">
             {% for quiz in season.quizzes %}