companion_radio: keep OLED on while externally powered 🤖🤖

[disq]

Summary

AUTO_OFF_MILLIS is a power-save feature aimed at battery use. When the board reports isExternalPowered() == true (USB or other DC source), blanking the screen serves no purpose — there's nothing to conserve. This PR makes the OLED stay on while externally powered, and starts the auto-off countdown from the moment power is removed (not from when it was applied).

Approach

Inside the _display->isOn() block of each companion UI flavour's loop(), refresh _auto_off whenever board.isExternalPowered() returns true:

```cpp
#if AUTO_OFF_MILLIS > 0
if (board.isExternalPowered()) {
_auto_off = millis() + AUTO_OFF_MILLIS;
}
if (millis() > _auto_off) {
_display->turnOff();
}
#endif
```

While USB-powered, _auto_off is pushed forward every tick so the existing turn-off branch never fires. The instant isExternalPowered() goes false, the refresh stops and the timer counts down naturally — giving a full fresh AUTO_OFF_MILLIS window before the screen blanks. No instantaneous-blank-on-unplug.

Applied to all three companion_radio UI flavours: ui-new, ui-tiny, ui-orig.

Safety

• MeshCore.h defines virtual bool isExternalPowered() { return false; } as the base-class default. Boards without an override compile to identical pre-change behaviour.
• NRF52Board overrides it only when NRF52_POWER_MANAGEMENT is defined (RAK3401, RAK4631, etc.).
• Existing battery-only deployments and any board that doesn't (yet) report external-power state are completely unaffected.

Why not also simple_repeater / simple_room_server / simple_sensor

Those examples target set-and-forget battery-powered deployments by convention. Easy follow-up if maintainers want the same behaviour there.

Testing

Verified on RAK3401 + companion_radio_ble:

• USB-powered: OLED stays on indefinitely past AUTO_OFF_MILLIS (15 s default).
• Unplug USB: OLED stays on for a fresh 15 s window from the unplug moment, then blanks as expected.
• Battery-only operation (no USB ever attached): existing behaviour — blanks 15 s after last button press.

🤖 Generated with Claude Code