feat: installer fetched from the direct address sets it as the agent's own with the main one as fallback (and downloads from it); agents fall back to whichever address is not theirs; (direct) beside the connection status

internal/server/handlers_agent.go

@@ -45,6 +45,7 @@ func (s *Server) handlePoll(w http.ResponseWriter, r *http.Request) {
 	}
 	// The agent sends its version and OS user occasionally; record them when
 	// present. Agents older than 0.0.2 send no user, so keep what we have.
+	s.viaDirectA.Store(a.ID, s.viaDirect(r))
 	v := r.Header.Get("X-Agent-Version")
 	u := firstNonEmpty(r.Header.Get("X-Agent-User"), a.RunUser)
 	if v != "" && (v != a.Version || u != a.RunUser) {
@@ -69,6 +70,7 @@ func (s *Server) setTunnelHeader(w http.ResponseWriter) {
 	}
 	if d := s.db.DirectAddress(); d != "" {
 		w.Header().Set(proto.FallbackHeader, d)
+		w.Header().Set(proto.MainHeader, s.db.HostAddress())
 	}
 }