Adds a LEDManager class (src/helpers/ui/LEDManager.h) that centralizes
all LED control into one component with begin()/loop() lifecycle and
per-pin active-HIGH/LOW polarity support.
LED settings are exposed as custom vars (led.status, led.activity)
accessible via companion radio binary protocol, CLI set/get commands,
and the SensorManager settings interface.
Status LED modes: off, boot-30s, slow blink (200ms/4s), always on.
Activity LED modes: off, BLE only, LoRa TX only, BLE + LoRa TX.
Integrated into 23 board variants, replacing scattered hardcoded
digitalWrite calls in onBeforeTransmit/onAfterTransmit/powerOff.
