refactor(admin): move auth_ids explode logic to model

- Remove auth_ids explode logic from Admin controller
- Add getAuthIdsAttr method to SystemAdmin model for auth_ids parsing
- This change improves code organization and reusability

app/admin/controller/system/Admin.php

@@ -99,7 +99,6 @@ class Admin extends AdminController
             }
             $save ? $this->success('保存成功') : $this->error('保存失败');
         }
-        $row->auth_ids = explode(',', $row->auth_ids ?: '');
         $this->assign('row', $row);
         return $this->fetch();
     }