ArduinoSerialInterface::isConnected() always returned true, so any
companion build with a USB interface believes a client is attached from
boot onwards. Two user visible effects:
- new message notifications never fire, because MyMesh only notifies
the UI (display, buzzer) while no client is connected
- on builds that also expose BLE the home screen shows '< Connected >'
instead of the BLE pairing PIN, so a freshly flashed device cannot be
paired at all
Add an optional connection check callback and wire it up per platform:
native USB-CDC (TinyUSB on nRF52/RP2040/ESP32 with USB_MODE=0) exposes
real DTR through (bool)Serial. The ESP32 USB-Serial-JTAG peripheral has
no DTR concept - it reports 'connected' as soon as the host enumerated
the device - so fall back to frame activity there. Plain UARTs keep the
previous assume-connected behaviour.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>