Show message on push notification

Currently, when users click on a push notification - the application is opened but the message is lost. In some situation if the message was longer, users might not be able to read it all. It would be nice to have a model on opening the app after clicking on a push notification that shows the full message.

查看應用程式目前如何處理推送通知，相關程式碼可能位於 service 檔案或主要進入點中。研究 Flutter 的本機通知或推送通知外掛程式，以顯示包含完整訊息 payload 的模態框。檢查現有的通知處理程式碼，了解應用程式開啟時訊息資料儲存在哪裡，或在哪裡遺失。

由索引模型根據 Issue 內容生成。