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/agent/client.go

@@ -319,7 +319,7 @@ func (a *Agent) pollOnce(ctx, reqCtx context.Context) error {
 		return err
 	}
 	a.applyTunnelHeader(ctx, resp.Header)
-	a.learnFallback(resp.Header.Get(proto.FallbackHeader))
+	a.learnAddrs(resp.Header.Get(proto.MainHeader), resp.Header.Get(proto.FallbackHeader))
 	a.applyTransport(resp.Header.Get(proto.TransportHeader))
 	a.dispatch(ctx, msg)
 	return nil
@@ -355,7 +355,7 @@ func (a *Agent) fetchConfig(ctx context.Context) bool {
 	}
 	defer resp.Body.Close()
 	a.applyTransport(resp.Header.Get(proto.TransportHeader))
-	a.learnFallback(resp.Header.Get(proto.FallbackHeader))
+	a.learnAddrs(resp.Header.Get(proto.MainHeader), resp.Header.Get(proto.FallbackHeader))
 	return a.applyTunnelHeader(ctx, resp.Header)
 }