port forward: the off switch gets a checkbox in Settings → General

`enable-port-forward-mux` was readable only by editing the config file.
It is a local setting of the controlling side, so it sits with the other
outgoing ones, after "Open connection in new tab", with a tooltip saying
what it does.

The two new keys are translated in every language. The three that the
mobile file manager added, "Export", "Export Logs" and "Import Folder",
were empty everywhere but five languages; they are filled in too, and
Korean's "xdp-portal-unavailable" with them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

flutter/lib/consts.dart

@@ -159,6 +159,7 @@ const String kOptionPeerTabVisible = "peer-tab-visible";
 const String kOptionPeerCardUiType = "peer-card-ui-type";
 const String kOptionCurrentAbName = "current-ab-name";
 const String kOptionEnableConfirmClosingTabs = "enable-confirm-closing-tabs";
+const String kOptionEnablePortForwardMux = "enable-port-forward-mux";
 const String kOptionAllowAlwaysSoftwareRender = "allow-always-software-render";
 const String kOptionEnableCheckUpdate = "enable-check-update";
 const String kOptionAllowAutoUpdate = "allow-auto-update";

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -509,6 +509,15 @@ class _GeneralState extends State<_General> {
           kOptionOpenNewConnInTabs,
           isServer: false,
         ),
+        Tooltip(
+          message: translate('port-forward-mux-tip'),
+          child: _OptionCheckBox(
+            context,
+            'Reuse one connection for port forwarding',
+            kOptionEnablePortForwardMux,
+            isServer: false,
+          ),
+        ),
         // though this is related to GUI, but opengl problem affects all users, so put in config rather than local
         if (isLinux)
           Tooltip(