Run port scan in the background and alert the user to servers exposed to WiFi network

[gnarea]

Is your feature request related to a problem? Please describe.

The user may have an app running an insecure server on 0.0.0.0 or the IP address of the device in its network (e.g., 192.168.X.Y), which could be exploited by other devices in the WiFi hotspot network.

Describe the solution you'd like

Run a port scan in the background, prioritising the most commonly used ports. We could ignore ports 0-1024 because only the system or an admin can listen on those ports.

This should be run each time the user is about to sync with private gateways, although it could also be run periodically or shortly after a new app has been installed (assuming we can be notified about that).

Related issues

Equivalent in desktop app: relaycorp/relaynet-courier#19