add enable-webrtc option; gate test_ipv6 under forced relay

OPTION_ENABLE_WEBRTC (hbb_common 48c2d4d) follows the udp/ipv6 punch
options end to end: default on against the public server, off against
private ones, same settings UI placement on desktop and mobile, and the
same bool2option local-option handling. Gates:

- controller: should_create_webrtc_offerer checks it first — no pc, no
  STUN/TURN gathering, no offer in the request;
- controlled: unlike the udp/ipv6 legs, which deliberately follow the
  request, answering builds a pc that gathers ICE from this host, so
  the answerer honors this machine's own switch too.

Translations for "Enable WebRTC P2P connection" added to all 50 lang
files next to the IPv6 entry (IPv6 and WebRTC are invariant terms in
the same grammatical slot in every one of them).

Also stop probing v6 reachability (test_ipv6) under any forced relay:
the v6 punch socket is never bound there, so the probe was wasted work
on every ws/proxy/relay connection.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ExUfAkYbq8UC9pQCiLy8TQ

flutter/lib/common.dart

@@ -1633,7 +1633,8 @@ String bool2option(String option, bool b) {
   String res;
   if (option.startsWith('enable-') &&
       option != kOptionEnableUdpPunch &&
-      option != kOptionEnableIpv6Punch) {
+      option != kOptionEnableIpv6Punch &&
+      option != kOptionEnableWebrtc) {
     res = b ? defaultOptionYes : 'N';
   } else if (option.startsWith('allow-') ||
       option == kOptionStopService ||

flutter/lib/consts.dart

@@ -176,6 +176,7 @@ const String kOptionEnableUdpPunch = "enable-udp-punch";
 const String kOptionEnableIpv6Punch = "enable-ipv6-punch";
 const String kOptionAllowSyncClipboardBetweenSessions =
     "allow-sync-clipboard-between-sessions";
+const String kOptionEnableWebrtc = "enable-webrtc";
 const String kOptionEnableTrustedDevices = "enable-trusted-devices";
 const String kOptionShowVirtualMouse = "show-virtual-mouse";
 const String kOptionVirtualMouseScale = "virtual-mouse-scale";

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -584,6 +584,12 @@ class _GeneralState extends State<_General> {
           kOptionEnableIpv6Punch,
           isServer: false,
         ),
+        _OptionCheckBox(
+          context,
+          'Enable WebRTC P2P connection',
+          kOptionEnableWebrtc,
+          isServer: false,
+        ),
         Tooltip(
           message: translate('sync-clipboard-between-sessions-tip'),
           child: _OptionCheckBox(

flutter/lib/mobile/pages/settings_page.dart

@@ -101,6 +101,7 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
   var _allowInsecureTlsFallback = false;
   var _disableUdp = false;
   var _enableIpv6Punch = false;
+  var _enableWebrtc = false;
   var _isUsingPublicServer = false;
   var _allowAskForNoteAtEndOfConnection = false;
   var _preventSleepWhileConnected = true;
@@ -143,6 +144,7 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
     _enableTrustedDevices = mainGetBoolOptionSync(kOptionEnableTrustedDevices);
     _enableUdpPunch = mainGetLocalBoolOptionSync(kOptionEnableUdpPunch);
     _enableIpv6Punch = mainGetLocalBoolOptionSync(kOptionEnableIpv6Punch);
+    _enableWebrtc = mainGetLocalBoolOptionSync(kOptionEnableWebrtc);
     _allowAskForNoteAtEndOfConnection =
         mainGetLocalBoolOptionSync(kOptionAllowAskForNoteAtEndOfConnection);
     _preventSleepWhileConnected =
@@ -841,6 +843,18 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
                 });
               },
             ),
+          if (!incomingOnly)
+            SettingsTile.switchTile(
+              title: Text(translate('Enable WebRTC P2P connection')),
+              initialValue: _enableWebrtc,
+              onToggle: (v) async {
+                await mainSetLocalBoolOption(kOptionEnableWebrtc, v);
+                final newValue = mainGetLocalBoolOptionSync(kOptionEnableWebrtc);
+                setState(() {
+                  _enableWebrtc = newValue;
+                });
+              },
+            ),
           SettingsTile(
               title: Text(translate('Language')),
               leading: Icon(Icons.translate),