nrf52: turn off BLE connection LED before SYSTEMOFF

Bluefruit's autoConnLed (on by default) drives LED_CONN. nRF52 SYSTEMOFF
only halts the core and leaves GPIO outputs latched in their last drive
state, so the BLE LED stayed lit through hibernation -- solid when a
companion app was connected, or caught mid-blink while advertising.

In the shared enterSystemOff() funnel, disable autoConnLed and stop
advertising so BLE no longer drives the pin, then pull LED_CONN low with
the polarity-aware ledOff() macro (respects each variant's LED_STATE_ON).
This covers every board that hibernates through enterSystemOff() --
rak3401, rak4631 and xiao_nrf52 -- in one place. The Bluefruit calls are
gated on the SoftDevice being enabled so the early low-voltage /
boot-protect path (before Bluefruit.begin()) is safe; ledOff() always runs.
Awake behavior is unchanged -- the LED still indicates BLE status.

Note: promicro and rak_wismesh_tag call sd_power_system_off() directly
rather than via enterSystemOff(), so they are not covered by this change.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/helpers/NRF52Board.cpp

@@ -156,6 +156,18 @@ void NRF52Board::enterSystemOff(uint8_t reason) {
     NRF_POWER->GPREGRET2 = reason;
   }
 
+  // Turn off the BLE connection LED before deep sleep. Bluefruit's autoConnLed
+  // (on by default) drives LED_CONN; nRF52 SYSTEMOFF only halts the core and
+  // leaves GPIO outputs latched in their last state, so the LED would otherwise
+  // stay lit through hibernation. Stop BLE re-driving the pin, then pull it low.
+  if (sd_enabled) {
+    Bluefruit.autoConnLed(false);   // stop BLE re-driving the LED
+    Bluefruit.Advertising.stop();   // stop advertising + its blink timer
+  }
+#if defined(LED_CONN) && (LED_CONN >= 0)
+  ledOff(LED_CONN);                 // polarity-aware (respects LED_STATE_ON)
+#endif
+
   // Flush serial buffers
   Serial.flush();
   delay(100);