feat: add restriction to prevent deletion of system tasks and rewards

- Implemented logic to hide delete button for system tasks and rewards in ItemList.vue, TaskView.vue, and RewardView.vue.
- Added backend checks in task_api.py and reward_api.py to return 403 for delete requests on system items.
- Ensured that items without a user_id are treated as system items across frontend and backend.
- Updated acceptance criteria to include UI and backend tests for the new functionality.

frontend/vue-app/src/layout/AuthLayout.vue

@@ -119,5 +119,6 @@ const showBack = computed(
   height: 0;
   overflow: hidden;
   overflow-y: visible;
+  padding: 1rem;
 }
 </style>