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.

src/MeshCore.h

@@ -35,6 +35,8 @@
 #define BRIDGE_DEBUG_PRINTLN(...) {}
 #endif
 
+class LEDManager;  // forward declaration
+
 namespace mesh {
 
 #define  BD_STARTUP_NORMAL     0  // getStartupReason() codes
@@ -42,6 +44,8 @@ namespace mesh {
 
 class MainBoard {
 public:
+  LEDManager* ledManager = nullptr;   // set by boards that have LEDs
+
   virtual uint16_t getBattMilliVolts() = 0;
   virtual float getMCUTemperature() { return NAN; }
   virtual bool setAdcMultiplier(float multiplier) { return false; };