[client/ui] Add async Up mode to avoid blocking profile switches

The daemon's Up RPC previously always blocked in waitForUp (up to 50s)
until the engine connected. The UI does not need this — status updates
already flow through the SubscribeStatus stream.

Add bool async = 4 to UpRequest. When true the daemon starts
connectWithRetryRuns and returns immediately; the CLI path (async=false,
the default) is unchanged.

ProfileSwitcher.SwitchActive now sets Async:true so all three RPCs
(Status, Switch, Down, Up) return quickly. The background goroutine and
its associated race condition are removed entirely.

client/server/server.go

@@ -747,6 +747,9 @@ func (s *Server) Up(callerCtx context.Context, msg *proto.UpRequest) (*proto.UpR
 	go s.connectWithRetryRuns(ctx, s.config, s.statusRecorder, s.clientRunningChan, s.clientGiveUpChan)
 
 	s.mutex.Unlock()
+	if msg.GetAsync() {
+		return &proto.UpResponse{}, nil
+	}
 	return s.waitForUp(callerCtx)
 }