Add centralized LEDManager for configurable LED behavior

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.

examples/simple_sensor/main.cpp

@@ -103,6 +103,7 @@ void setup() {
   command[0] = 0;
 
   sensors.begin();
+  if (board.ledManager) sensors.setLEDManager(board.ledManager);
 
   the_mesh.begin(fs);
 
@@ -143,6 +144,7 @@ void loop() {
 
   the_mesh.loop();
   sensors.loop();
+  if (board.ledManager) board.ledManager->loop();
 #ifdef DISPLAY_CLASS
   ui_task.loop();
 #endif