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/components/auth/Signup.vue

@@ -104,7 +104,7 @@
       <ErrorMessage v-if="signupError" :message="signupError" aria-live="polite" />
 
       <!-- Modal for "Account already exists" -->
-      <ModalDialog v-if="showEmailExistsModal">
+      <ModalDialog v-if="showEmailExistsModal" :imageUrl="undefined">
         <h3>Account already exists</h3>
         <p>
           An account with <strong>{{ email }}</strong> already exists.
@@ -156,7 +156,7 @@ import { EMAIL_EXISTS, MISSING_FIELDS } from '@/common/errorCodes'
 import '@/assets/view-shared.css'
 import '@/assets/actions-shared.css'
 import '@/assets/button-shared.css'
-import '@/assets/global.css'
+import '@/assets/colors.css'
 import '@/assets/edit-forms.css'
 
 const router = useRouter()