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.

assets/styles/backoffice.scss

@@ -1,3 +1,5 @@
 .col-result-xs { width: 10%; }
 .col-result-sm { width: 15%; }
 .col-result-md { width: 20%; }
+
+.modal-content > turbo-frame { display: contents; }