feat: add bank question via modal + dirty modal guard

- Add question in question bank now opens a modal instead of navigating
  to a full-page form, consistent with the edit modal pattern
- Modal closes are blocked by static backdrop once the user has made any
  change (input, checkbox, drag-reorder, sort, randomize, remove answer)
- Dirty state resets when the modal is fully hidden

assets/controllers/bo/modal_form_controller.js

@@ -23,9 +23,29 @@ export default class extends Controller {
     const res = await fetch(url, {headers: {'X-Modal-Request': '1'}});
     this.modalBodyTarget.innerHTML = await res.text();
     this._moveFooter();
+    this._bindDirty();
     this._bindForm(url);
   }
 
+  _bindDirty() {
+    const modal = Modal.getOrCreateInstance(this.modalTarget);
+    modal._config.backdrop = true;
+    modal._config.keyboard = true;
+
+    const markDirty = () => {
+      modal._config.backdrop = 'static';
+      modal._config.keyboard = false;
+    };
+
+    this.modalBodyTarget.addEventListener('input', markDirty, {once: true});
+    this.modalBodyTarget.addEventListener('change', markDirty, {once: true});
+
+    this.modalTarget.addEventListener('hidden.bs.modal', () => {
+      modal._config.backdrop = true;
+      modal._config.keyboard = true;
+    }, {once: true});
+  }
+
   _moveFooter() {
     if (!this.hasModalFooterTarget) return;
     const template = this.modalBodyTarget.querySelector('template[data-modal-footer]');