Make the tray Exit Node submenu selectable (mutually exclusive, sourced from
ListNetworks by NetID) instead of read-only.
Add networksRevision to the status snapshot, bumped by the route manager on
network-map and selection changes, so the tray and the React NetworksContext
re-fetch ListNetworks via the push stream instead of polling. The peer-status
route list only carries chosen routes, so a candidate exit node appearing or
disappearing would otherwise never reach the UI.
f693d268b4