'Install app promotion': put it in app settings, only show on relevant pages

- Install app promotion is displayed as a toast message if it is not yet installed

- User can dismiss popup but this should be available in settings afterwards if app is not yet installed

Also, the Toast should NOT be displayed if the user is not logged in. It distracts and confuses potential outside visitors.

- See event handler in layout.svelte

- Modify logic this way:

- Users logs in --> install app appears after 10 seconds IF not already installed