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/dist/assets/lock-Lq5iEyDR.js

@@ -0,0 +1,6 @@
+import{c as e}from"./index-CvsGOA4u.js";/**
+ * @license lucide-vue-next v1.0.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */const t=e("lock",[["rect",{width:"18",height:"11",x:"3",y:"11",rx:"2",ry:"2",key:"1w4ew1"}],["path",{d:"M7 11V7a5 5 0 0 1 10 0v4",key:"fwvmzm"}]]);export{t as L};