[Demo app] The authgear config should persist after quitting app

## Problem

After quitting the app, the config is cleared. And the user need to fill in the info again.

## Expect

The configuration should retain after quitting and reopen, and it should remain "Configured"

Locate the Flutter demo app's Authgear configuration entry point and trace how its values are loaded and cleared across app restarts. Reproduce the issue by configuring the app, quitting, and reopening it. Done means the configuration is retained and the app still shows "Configured" after reopening.

Written by the indexing model from the issue text.