chatgui: IPv4+IPv6 dual-stack listen by default

- Default config creates [server:lan] (0.0.0.0) + [server:lan6] ([::])
- Network settings save also writes [server:lan6] section
- Sets IPV6_V6ONLY=1 on IPv6 sockets to avoid conflict with IPv4
- interface_addr for [::] bind prefers permanent IPv6 over temporary

tools/chatgui/src/networksettingspage.cpp

@@ -140,6 +140,11 @@ void NetworkSettingsPage::saveToConfig(const QString& configPath) const {
     config_update_option(configPath, SECTION, "transport",
                          m_protoCombo->currentText().toLower(),
                          CONFIG_UPDATE_REPLACE);
+
+    // IPv6 dual-stack: always add [server:lan6] with same port
+    config_update_option(configPath, "server:lan6", "addr",
+                         QString("[::]:%1").arg(m_portSpin->value()),
+                         CONFIG_UPDATE_REPLACE);
 }
 
 void NetworkSettingsPage::onInterfaceChanged(int index) {