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/backoffice.js

@@ -1,6 +1,8 @@
 import 'bootstrap/dist/css/bootstrap.min.css';
 import 'bootstrap-icons/font/bootstrap-icons.min.css';
 import './styles/backoffice.scss';
+import {Turbo} from '@hotwired/turbo';
+Turbo.session.drive = false;
 import './stimulus.js';
 import './bootstrap.js';
 import * as Sentry from '@sentry/browser';