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/internal/routemanager/manager.go

@@ -439,6 +439,11 @@ func (m *DefaultManager) UpdateRoutes(
 
 		m.updateClientNetworks(updateSerial, filteredClientRoutes)
 		m.notifier.OnNewRoutes(filteredClientRoutes)
+		// A new network map can add or drop route/exit-node candidates without
+		// touching any peer's chosen-route state, so the peer status alone
+		// wouldn't notify SubscribeStatus subscribers. Bump the revision so the
+		// UI re-fetches ListNetworks.
+		m.statusRecorder.BumpNetworksRevision()
 	}
 	m.clientRoutes = clientRoutes
 
@@ -579,6 +584,10 @@ func (m *DefaultManager) TriggerSelection(networks route.HAMap) {
 	if err := m.stateManager.UpdateState((*SelectorState)(m.routeSelector)); err != nil {
 		log.Errorf("failed to update state: %v", err)
 	}
+
+	// A selection change flips Network.selected without altering the candidate
+	// set, so bump the revision to push the new state to the UI.
+	m.statusRecorder.BumpNetworksRevision()
 }
 
 // stopObsoleteClients stops the client network watcher for the networks that are not in the new list