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/styles.css

@@ -0,0 +1,100 @@
+/*buttons*/
+.btn {
+  font-weight: 600;
+  border: none;
+  border-radius: 8px;
+  padding: 0.7rem 1.5rem;
+  font-size: 1.1rem;
+  cursor: pointer;
+  box-shadow: 0 2px 8px rgba(102, 126, 234, 0.08);
+  transition:
+    background 0.18s,
+    color 0.18s;
+  display: inline-block;
+}
+
+/* Primary button (e.g., Save, Confirm) */
+.btn-primary {
+  background: var(--btn-primary);
+  color: #fff;
+}
+.btn-primary:hover,
+.btn-primary:focus {
+  background: var(--btn-primary-hover);
+}
+.btn-primary:disabled,
+.btn-primary[disabled] {
+  background: var(--btn-secondary, #f3f3f3);
+  color: var(--btn-secondary-text, #666);
+  cursor: not-allowed;
+  opacity: 0.7;
+}
+
+/* Secondary button (e.g., Cancel) */
+.btn-secondary {
+  background: var(--btn-secondary);
+  color: var(--btn-secondary-text);
+}
+.btn-secondary:hover,
+.btn-secondary:focus {
+  background: var(--btn-secondary-hover);
+}
+
+/* Danger button (e.g., Delete) */
+.btn-danger {
+  background: var(--btn-danger);
+  color: #fff;
+}
+.btn-danger:hover,
+.btn-danger:focus {
+  background: var(--btn-danger-hover);
+}
+
+/* Green button (e.g., Confirm) */
+.btn-green {
+  background: var(--btn-green);
+  color: #fff;
+}
+.btn-green:hover,
+.btn-green:focus {
+  background: var(--btn-green-hover);
+}
+
+/* Link-style button */
+.btn-link {
+  color: var(--btn-primary);
+  text-decoration: underline;
+  background: none;
+  border: none;
+  padding: 0;
+  cursor: pointer;
+  font-weight: 600;
+  margin-left: 6px;
+}
+.btn-link:disabled {
+  text-decoration: none;
+  opacity: 0.75;
+  cursor: default;
+  pointer-events: none;
+  color: var(--btn-primary);
+}
+
+/* Rounded button */
+.round-btn {
+  background: var(--sign-in-btn-bg);
+  color: var(--sign-in-btn-color);
+  border: 2px solid var(--sign-in-btn-border);
+  border-radius: 6px;
+  font-size: 0.85rem;
+  font-weight: 600;
+  padding: 0.2rem 0.5rem;
+  margin-right: 0.1rem;
+  cursor: pointer;
+  transition:
+    background 0.18s,
+    color 0.18s;
+}
+.round-btn:hover {
+  background: var(--sign-in-btn-hover-bg);
+  color: var(--sign-in-btn-hover-color);
+}