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/i18n/locales/en.ts

@@ -1042,6 +1042,10 @@ export default {
       createGroup: 'Create Group',
       editGroup: 'Edit Group',
       deleteGroup: 'Delete Group',
+      sortOrder: 'Sort',
+      sortOrderHint: 'Drag groups to adjust display order, groups at the top will be displayed first',
+      sortOrderUpdated: 'Sort order updated',
+      failedToUpdateSortOrder: 'Failed to update sort order',
       allPlatforms: 'All Platforms',
       allStatus: 'All Status',
       allGroups: 'All Groups',