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.

backend/app/services/groups.py

@@ -95,3 +95,7 @@ class GroupStore:
         if not found:
             raise ValueError("persona not found")
         return self.groups.replace(gid, group)
+
+    def delete(self, gid: str) -> None:
+        """Hard-delete a group (personas/report included)."""
+        self.groups.delete(gid)