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/controllers/bo/modal_controller.js

@@ -0,0 +1,48 @@
+import {Controller} from '@hotwired/stimulus';
+import {Modal} from 'bootstrap';
+
+export default class extends Controller {
+  static targets = ['modal', 'frame'];
+
+  open(event) {
+    event.preventDefault();
+    const {src, modalTitle} = event.currentTarget.dataset;
+    if (modalTitle) {
+      const titleEl = this.modalTarget.querySelector('.modal-title');
+      if (titleEl) titleEl.textContent = modalTitle;
+    }
+    this._resetDirty();
+    this.frameTarget.innerHTML = '<div class="modal-body text-center py-4"><div class="spinner-border" role="status"></div></div>';
+    this.frameTarget.removeAttribute('src');
+    this.frameTarget.setAttribute('src', src);
+    Modal.getOrCreateInstance(this.modalTarget).show();
+  }
+
+  frameLoad() {
+    this._bindDirty();
+  }
+
+  frameSubmitEnd(event) {
+    if (event.detail.success) {
+      Modal.getOrCreateInstance(this.modalTarget).hide();
+      window.location.reload();
+    }
+  }
+
+  _bindDirty() {
+    const modal = Modal.getOrCreateInstance(this.modalTarget);
+    const markDirty = () => {
+      modal._config.backdrop = 'static';
+      modal._config.keyboard = false;
+    };
+    this.frameTarget.addEventListener('input', markDirty, {once: true});
+    this.frameTarget.addEventListener('change', markDirty, {once: true});
+    this.modalTarget.addEventListener('hidden.bs.modal', () => this._resetDirty(), {once: true});
+  }
+
+  _resetDirty() {
+    const modal = Modal.getOrCreateInstance(this.modalTarget);
+    modal._config.backdrop = true;
+    modal._config.keyboard = true;
+  }
+}