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/ui-new/UITask.cpp

@@ -13,6 +13,13 @@
 #ifndef AUTO_OFF_MILLIS
   #define AUTO_OFF_MILLIS     15000   // 15 seconds
 #endif
+// Upstream MeshCore version, shown on the splash screen. Most variants set it in
+// their platformio.ini; the ones that don't used to fail to compile this file
+// outright rather than fall back, which quietly made every ui-new env on those
+// boards unbuildable (heltec v3/v4, thinknode m1/m5, mesh pocket, techo).
+#ifndef MESHCORE_VERSION
+  #define MESHCORE_VERSION    "1.16"
+#endif
 #define BOOT_SCREEN_MILLIS   3000   // 3 seconds
 
 #ifdef PIN_STATUS_LED