Refactor components to use ModalDialog and StatusMessage; update styles and remove unused files

- Replaced inline modal dialogs in ParentView with a reusable ModalDialog component.
- Introduced StatusMessage component for loading and error states in ParentView.
- Updated styles to use new colors.css and styles.css for consistent theming.
- Removed ChildRewardList.vue and ChildTaskList.vue components as they were no longer needed.
- Adjusted RewardAssignView and TaskAssignView to use new styles and shared button styles.
- Cleaned up imports across components to reflect the new styles and removed unused CSS files.

frontend/vue-app/src/assets/view-shared.css

@@ -41,20 +41,6 @@
   text-align: center;
 }
 
-/* Dialog Message */
-.dialog-message {
-  font-size: 1.08rem;
-  color: var(--dialog-message);
-  font-weight: 500;
-  margin-bottom: 1.2rem;
-  text-align: center;
-}
-.dialog-message .child-name {
-  color: var(--dialog-child-name);
-  font-weight: 700;
-  margin-left: 2px;
-}
-
 /* Info Sections (Reward/Task) */
 .info,
 .reward-info,