feat(helpdesk): add domain linking, bulk actions, and revision polish to handovers

Adds domain selection (cascaded from debitor) to handover creation and edit,
bulk delete with confirmation dialog, and various UI improvements to the
handover wizard and list page. Includes migration for domain columns,
domain-select AJAX endpoint, and updated tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/web/js/handover-schema-editor.js

@@ -131,6 +131,17 @@ function init(root) {
         (t.move_down || 'Move down') + ' ' + (t.field_label || 'field') + ' ' + (index + 1),
       ));
     }
+    actions.appendChild(createIconButton(
+      'bi-copy', 'secondary outline small',
+      () => {
+        const clone = { ...field, label: field.label, key: field.key || '' };
+        if (field.options) { clone.options = field.options.map((o) => ({ ...o })); }
+        fields.splice(index + 1, 0, clone);
+        render();
+        notifyChange();
+      },
+      (t.duplicate_field || 'Duplicate field') + ' ' + (index + 1),
+    ));
     actions.appendChild(createIconButton(
       'bi-trash', 'danger outline small',
       () => { fields.splice(index, 1); render(); notifyChange(); },