feat(heltec): solo dual-transport builds for Heltec V3/V4

Port the Wio Tracker L1 solo firmware (full on-device UI, dual BLE/USB
companion transport) to Heltec V3 and V4 OLED boards. Neither board has
a joystick or CardKB on-board, so each new env wires up both as optional
peripherals with default pins from what the board leaves free, gated
behind the existing UI_HAS_JOYSTICK/ENV_PIN_SDA+SCL flags.

DUAL_SERIAL was nRF52-only; added an ESP32 helpers/esp32/DualSerialInterface.h
counterpart so the flag isn't silently ignored on these boards. On V4's
native USB CDC, isClientConnected() also honours (bool)Serial (real DTR),
same as the nRF52 version; V3 has no native CDC so it stays BLE-only there.

Screen (SDA 17/SCL 18) and CardKB (SDA 3/SCL 4) confirmed working on real
V4 hardware.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

examples/companion_radio/main.cpp

@@ -41,6 +41,9 @@ static uint32_t _atoi(const char* sp) {
     #ifndef TCP_PORT
       #define TCP_PORT 5000
     #endif
+  #elif defined(DUAL_SERIAL)
+    #include <helpers/esp32/DualSerialInterface.h>
+    DualSerialInterface serial_interface;
   #elif defined(BLE_PIN_CODE)
     #include <helpers/esp32/SerialBLEInterface.h>
     SerialBLEInterface serial_interface;
@@ -220,6 +223,8 @@ void setup() {
 
   WiFi.begin(WIFI_SSID, WIFI_PWD);
   serial_interface.begin(TCP_PORT);
+#elif defined(DUAL_SERIAL)
+  serial_interface.begin(BLE_NAME_PREFIX, the_mesh.getNodePrefs()->node_name, the_mesh.getBLEPin(), Serial);
 #elif defined(BLE_PIN_CODE)
   serial_interface.begin(BLE_NAME_PREFIX, the_mesh.getNodePrefs()->node_name, the_mesh.getBLEPin());
 #elif defined(SERIAL_RX)