feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

frontend/src/api/admin/groups.ts

@@ -153,6 +153,20 @@ export async function getGroupApiKeys(
   return data
 }
 
+/**
+ * Update group sort orders
+ * @param updates - Array of { id, sort_order } objects
+ * @returns Success confirmation
+ */
+export async function updateSortOrder(
+  updates: Array<{ id: number; sort_order: number }>
+): Promise<{ message: string }> {
+  const { data } = await apiClient.put<{ message: string }>('/admin/groups/sort-order', {
+    updates
+  })
+  return data
+}
+
 export const groupsAPI = {
   list,
   getAll,
@@ -163,7 +177,8 @@ export const groupsAPI = {
   delete: deleteGroup,
   toggleStatus,
   getStats,
-  getGroupApiKeys
+  getGroupApiKeys,
+  updateSortOrder
 }
 
 export default groupsAPI