tray: selectable exit nodes + push-based network list refresh

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.

client/proto/daemon.proto

@@ -443,6 +443,12 @@ message FullStatus {
 
   bool lazyConnectionEnabled = 9;
   SSHServerState sshServerState = 10;
+
+  // networksRevision bumps whenever the set of routed networks (route and
+  // exit-node candidates) or their selected state changes. The UI fingerprints
+  // on it to know when to re-fetch ListNetworks via the push stream, instead
+  // of polling on every status snapshot.
+  uint64 networksRevision = 11;
 }
 
 // Networks