Show feedback when timetable auto-saves

## Description
Currently, when the timetable auto-saves, there is no visual feedback to inform the user whether the save was successful or if an error occurred. Adding feedback messages will improve user experience and make it clear that data is being saved in real-time.

## Steps to Implement
1. Detect when the auto-save function is triggered.
2. Show a temporary message (e.g., “Saved!”) on successful save.
3. Display an error message (e.g., “Error saving, please try again”) if the save fails.
4. Consider using toast notifications, banners, or small inline messages.
5. Ensure the message disappears after a short duration (e.g., 2–3 seconds).

## Expected Behavior
- Users receive clear feedback every time the timetable auto-saves.
- Success messages appear for successful saves, and error messages appear for failed saves.
- Messages do not obstruct the timetable UI.

在前端程式碼中尋找自動儲存功能，它可能位於處理課表編輯的元件中。使用 UI 函式庫或自訂元件實作 toast 或內嵌通知。透過觸發儲存並確認訊息會出現，且在 2-3 秒後消失而不會阻塞 UI 來進行測試。

由索引模型根據 Issue 內容生成。