NetworkService: use NTP server from DHCP Option 42 when available

Call sntp_servermode_dhcp(1) before WiFi.begin() so that lwIP's DHCP
client passes any NTP server address received in DHCP Option 42 to the
SNTP client (slot 0).

Replace configTzTime() with explicit setenv/tzset + sntp_setservername
+ sntp_init() to avoid overwriting the DHCP-provided server.  Static
fallback servers (0.ru.pool.ntp.org, ntp.ix.ru, ntp21.vniiftri.ru) are
placed in slots 1-2 when DHCP provides a server, or slots 0-2 otherwise.

This relies on CONFIG_LWIP_DHCP_GET_NTP_SRV=y, which is already enabled
in the pre-built Arduino ESP32 core (framework-arduinoespressif32).

Add debug logging of the initialised NTP server list, guarded by
WIFI_DEBUG_LOGGING.

src/helpers/NetworkService.cpp

@@ -282,6 +282,7 @@ void NetworkService::ensureWifi(bool network_required) {
   if (!_wifi_started) {
     WiFi.mode(WIFI_STA);
     WiFi.setSleep(toEspPowerSave(_prefs.wifi_powersave));
+    sntp_servermode_dhcp(1);
     _wifi_started = true;
   }
 
@@ -297,7 +298,34 @@ void NetworkService::updateTimeSync() {
   }
 
   if (!_sntp_started) {
-    configTzTime("UTC0", "0.ru.pool.ntp.org", "ntp.ix.ru", "ntp21.vniiftri.ru");
+    setenv("TZ", "UTC0", 1);
+    tzset();
+
+    const ip_addr_t* dhcp_srv = sntp_getserver(0);
+    bool have_dhcp_ntp = dhcp_srv && !ip_addr_isany(dhcp_srv);
+
+    if (have_dhcp_ntp) {
+        sntp_setservername(1, "0.ru.pool.ntp.org");
+        sntp_setservername(2, "ntp.ix.ru");
+    } else {
+        sntp_setservername(0, "0.ru.pool.ntp.org");
+        sntp_setservername(1, "ntp.ix.ru");
+        sntp_setservername(2, "ntp21.vniiftri.ru");
+    }
+
+    sntp_init();
+
+#if WIFI_DEBUG_LOGGING && ARDUINO
+    for (int i = 0; i < SNTP_MAX_SERVERS; i++) {
+        const ip_addr_t* srv = sntp_getserver(i);
+        const char* name = sntp_getservername(i);
+        if (name)
+            Serial.printf("[NTP] server[%d] = %s\n", i, name);
+        else if (srv && !ip_addr_isany(srv))
+            Serial.printf("[NTP] server[%d] = %s (DHCP)\n", i, ipaddr_ntoa(srv));
+    }
+#endif
+
     _sntp_started = true;
   }