refactor: separate NAT detection from topology into standalone nat_detection module

- Extract NAT detection logic (~250 lines) from topo_group.c into new nat_detection.c
- NAT_DETECTION struct owns ping_pending, next_ping_req_id, allow_nat_check_local (removed from TOPO_GROUP)
- route_ping.* signatures changed: TOPO_GROUP* -> NAT_DETECTION* (+ TOPO_GROUP* where needed)
- topo_group.c now only dispatches BGP subcommands to nat_detection_* handlers
- UTUN_INSTANCE gets struct NAT_DETECTION* nat_det field (init/destroy in topo_groups_init/destroy)
- External callers (etcp_connections, etcp, control_server) call nat_detection_* directly
- nat_detection_cancel_for_conn / nat_detection_send_nat_info / nat_detection_link_ready / nat_detection_request_check_all
- route_ping_destroy_pending properly implemented (was dead code)
- TOPO_SUBCMD_PING_REQ/RESP moved from route_ping.h to topo_group.h (all BGP subcommands in one place)
- Tests updated: test_nat_detection, test_route_ping - both PASSED
- Build: auto(CORE_SOURCES+libutun) and cmake(chatgui/libutun)

tools/chatgui/src/settingsdialog.cpp

@@ -3,6 +3,7 @@
 #include "networksettingspage.h"
 #include "databasesettingspage.h"
 #include "statuspage.h"
+#include "nodespage.h"
 #include "../transport/gui_bridge.h"
 #include "../../lib/mem.h"
 #include "../db/db_manager.h"
@@ -23,7 +24,7 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     , m_configPath(configPath)
 {
     setWindowTitle("Settings");
-    setMinimumSize(560, 460);
+    setMinimumSize(760, 460);
     setModal(true);
 
     auto* mainLayout = new QHBoxLayout(this);
@@ -42,6 +43,7 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     m_categoryList->addItem("Network");
     m_categoryList->addItem("Database");
     m_categoryList->addItem("Status");
+    m_categoryList->addItem("Nodes");
     mainLayout->addWidget(m_categoryList);
 
     // --- Right panel: stacked pages ---
@@ -77,6 +79,10 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     // Status page
     m_statusPage = new StatusPage(m_pages);
     m_pages->addWidget(m_statusPage);
+
+    // Nodes page
+    m_nodesPage = new NodesPage(db, m_pages);
+    m_pages->addWidget(m_nodesPage);
     rightLayout->addWidget(m_pages, 1);
 
     // --- Bottom buttons ---
@@ -134,6 +140,7 @@ void SettingsDialog::loadProfileFromConfig(const QString& path) {
 void SettingsDialog::onCategoryChanged(int row) {
     m_pages->setCurrentIndex(row);
     if (row == 3) m_statusPage->refreshStatus();
+    if (row == 4) m_nodesPage->refreshNodes();
 }
 
 void SettingsDialog::onSave() {