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/shared/StatusMessage.vue

@@ -0,0 +1,32 @@
+<script setup lang="ts">
+defineProps<{
+  loading: boolean
+  error: string | null
+}>()
+</script>
+
+<template>
+  <div v-if="loading" class="loading">Loading...</div>
+  <div v-else-if="error" class="error">Error: {{ error }}</div>
+</template>
+
+<style scoped>
+.loading {
+  color: var(--loading-color);
+  min-height: 200px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  text-align: center;
+}
+
+.error {
+  color: var(--error-color);
+  min-height: 100px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  text-align: center;
+  padding: 1rem;
+}
+</style>