improve: Restore TextField focus state after configuration change

[satwanjyu]

Current implementation loses TextField focus after configuration change. The cursor location (selection) is preserved via rememberSaveable, but that wouldn't be helpful if the focus is lost and the customer will have to click into the TextField which would then point the cursor to a new location.

Putting this in the backlog since I haven't found a trivial way to restore focus state (esp. for a large form).

[satwanjyu]

I've just realized rememberSaveable<TextFieldValue> wouldn't work since configuration change includes composition change e.g. rotating to landscape changes the composition to NavRailScaffold. I'd need to preserve TextFieldState in ViewModel.

Related to #16