Home network: compare both machines' networks over the relay #219

Follow-up to #216 / PR #218 (decision #123). One machine alone can't tell "the other machine is on a different Wi-Fi", "its VPN is on", "Home network isn't picked there" or "a firewall drops what we send". Its linked machines can.

Proposal

Each machine keeps a small sealed net/<machine id> object on the relay saying:

• whether Home network sync is on and listening,
• its home-network subnets (network address and prefix only, e.g. 192.168.1.0/24),
• whether its broadcasts leave through a VPN or virtual adapter,
• which linked machines it hears right now (by id).

It's sent when any of that changes (at most every 30 s) and, only while a linked machine is missing or can't hear this one, every 5 minutes so the others know it's still running. Nothing is sent while everything works.

The other machines use it to say, per missing machine:

• Home network sync isn't on there
• it hasn't checked in since … (asleep, Patch Pilot closed, offline)
• a VPN is on there
• it's on a different network
• it hears this machine but nothing from it arrives here (firewall here)
• same network, both listening, neither hears: firewall or the router keeping devices apart (guest Wi-Fi / AP isolation)

…and, on a machine that is heard, "Laptop can't hear this machine, so this machine's files aren't reaching it".

A cause is named only when the other machine's report is fresh (under 12 minutes old) and agrees.