[Feature/Fix] Add Power-Off / Deep Sleep support for Elecrow ThinkNode M3

[Balou1986]

Description

Currently, on the Elecrow ThinkNode M3 (nRF52840), there is no way to power down or put the device into deep sleep using either the hardware button or the MeshCore Companion App.

• Hardware Button: Long-pressing the button (5–6 seconds) triggers a beep and turns on the blue LED (entering Bluetooth pairing mode / reboot), but does not execute a system shutdown.
• Companion App: The app only provides a restart/reboot option for this device target, with no option to power off.

Solution idea

• Implement proper button-mapping for the ThinkNode M3 target so that a long-press (e.g., 5 seconds) invokes the nrf_power_system_off() / deep sleep routine to turn off the device.
• Add power-down / soft-shutdown command support for the M3 target in the firmware so it can be initiated via the Companion App interface.

Device & Firmware Info

• Device: Elecrow ThinkNode M3 (nRF52840 + SX1262/LR1110)
• Firmware Version: 1.16.0