🐛 fix(classroom): replace error message state with toast notification

- 修改消息发送失败的错误提示方式，从设置errorMessage状态改为调用showToast显示错误提示
- 提升用户体验，使错误提示更明显且不占用界面空间

src/client/mobile/components/Classroom/useClassroom.ts

@@ -637,7 +637,7 @@ export const useClassroom = ({ user }:{ user : User }) => {
       setMsgText('');
       setErrorMessage('');
     } catch (err: any) {
-      setErrorMessage(`消息发送失败: ${err.message}`);
+      showToast('error', `消息发送失败: ${err.message}`);
     }
   };