feat(training): delete group button (admin only) on Training page

- Backend DELETE /api/groups/<gid> (admin, own-org enforced via _get_owned_group);
  cascades removal of the group's sessions. GroupStore.delete added.
- Training.vue: each group card gets a trash (Trash2 line icon) delete button shown only
  to admins (auth.isAdmin); confirm dialog; @click.stop so it doesn't navigate. Card
  restructured so the delete button is not inside the clickable router-link.
- test_user_journey: trainee cannot delete (403), admin deletes, group gone (404).
All suites pass. Rebuilt dist.

frontend/src/api/index.js

@@ -43,6 +43,7 @@ export const api = {
   createGroup: (formData) => request('POST', '/api/groups', formData, true),
   listGroups: () => request('GET', '/api/groups'),
   getGroup: (id) => request('GET', `/api/groups/${id}`),
+  deleteGroup: (id) => request('DELETE', `/api/groups/${id}`),
   analyzeGroup: (id, opts = {}) => request('POST', `/api/groups/${id}/analyze${opts.append ? '?append=true' : ''}`),
   listPersonas: (gid) => request('GET', `/api/groups/${gid}/personas`),
   getPersona: (gid, pid) => request('GET', `/api/groups/${gid}/personas/${pid}`),