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/sparkles-BfSk-aam.js

@@ -0,0 +1,6 @@
+import{c as a}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 l=a("sparkles",[["path",{d:"M11.017 2.814a1 1 0 0 1 1.966 0l1.051 5.558a2 2 0 0 0 1.594 1.594l5.558 1.051a1 1 0 0 1 0 1.966l-5.558 1.051a2 2 0 0 0-1.594 1.594l-1.051 5.558a1 1 0 0 1-1.966 0l-1.051-5.558a2 2 0 0 0-1.594-1.594l-5.558-1.051a1 1 0 0 1 0-1.966l5.558-1.051a2 2 0 0 0 1.594-1.594z",key:"1s2grr"}],["path",{d:"M20 2v4",key:"1rf3ol"}],["path",{d:"M22 4h-4",key:"gwowj6"}],["circle",{cx:"4",cy:"20",r:"2",key:"6kqj1y"}]]);export{l as S};