Add an `account` dimension (Apple/Google `sub`, or any shared id) advertised
over mDNS alongside the device name. The LAN transport now connects to a peer
when they share a room OR a non-empty account — so your own devices find each
other and sync on the network regardless of room, no pairing step. Peer gains
`account` so the UI can mark "your devices".
Engine::new gains `account` (6th arg, ""=signed out); all callers updated.
Apps pass "" until sign-in lands; agent takes --account.
Proven by examples/account_pair.rs: two devices in DIFFERENT rooms with the
same account sync over LAN with no server.
Next: derive the account from Sign in with Apple / Google so this is automatic
(the cross-network version = account-derived shared server room).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Patrick Ecord
c3b9b7940f
