Indicate network connectivity in User UI

[mjuchli-da]

The WG User UI currently shows a green dot in the header next to the network, and similarly, shows "connected" in the networks listing, even if the network connectivity to the ledger was not established.

It appears that we render the user ui (see index.ts) app-layout and thus app-header with the wrong parameters. The user api endpoint listSessions should be used to determine the true network connectivity status.