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

backend/internal/handler/admin/admin_service_stub_test.go

@@ -357,5 +357,9 @@ func (s *stubAdminService) GetUserBalanceHistory(ctx context.Context, userID int
 	return s.redeems, int64(len(s.redeems)), 100.0, nil
 }
 
+func (s *stubAdminService) UpdateGroupSortOrders(ctx context.Context, updates []service.GroupSortOrderUpdate) error {
+	return nil
+}
+
 // Ensure stub implements interface.
 var _ service.AdminService = (*stubAdminService)(nil)