Address shell component of: phcode-dev/phoenix#2149

1. We cannot use the visibility: false logic as suggested in the root Tauri issue: Allow changing the webview background color  tauri-apps/tauri#1564 as it lead to add load time delay perception of up to 200ms(Though the actual load time is not affected, just that we hide the main window until black paint is done.) . This will affect the light weight perception of the app and hence cannot be the default.

So as a middle ground, adding optional support so that users can selectively trade-off delayed startup till the dark theme is rendered or faster show of app window.

The impl reads an existing boot_config.json file in app config dir and also adds fault tolarance to determine if to hide all on start. Its set to false by default and can be set to true from phcode settings.