companion_radio: opt-in KEEP_DISPLAY_ON_USB to keep OLED on while powered

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.

But OLEDs are vulnerable to burn-in with static content, so this
behaviour is gated behind a new build flag KEEP_DISPLAY_ON_USB. Default
is unchanged from upstream — the display blanks after AUTO_OFF_MILLIS
on USB or battery. Variants that ship with an LCD instead of an OLED
(e.g. heltec_t096) can opt in by adding -D KEEP_DISPLAY_ON_USB to
their env, gaining always-on-while-powered without exposing OLED users
to burn-in risk.

When the flag is enabled, the implementation refreshes _auto_off every
loop iteration while externally powered, so the timer naturally counts
a fresh AUTO_OFF_MILLIS window from the moment power is removed —
no instantaneous-blank-on-unplug.

Applied to all three companion_radio UI flavours (ui-new, ui-tiny,
ui-orig). Boards without an isExternalPowered() override use the
base-class default in MeshCore.h (returns false), so battery-powered
behaviour is unchanged everywhere.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

examples/companion_radio/ui-new/UITask.cpp

@@ -809,6 +809,15 @@ void UITask::loop() {
       _display->endFrame();
     }
 #if AUTO_OFF_MILLIS > 0
+#ifdef KEEP_DISPLAY_ON_USB
+    // Opt-in: refresh the auto-off deadline while externally powered, so the
+    // timer counts from the moment external power is removed. Off by default
+    // because OLED panels burn in quickly; only enable for LCD targets or
+    // where the display is replaceable.
+    if (board.isExternalPowered()) {
+      _auto_off = millis() + AUTO_OFF_MILLIS;
+    }
+#endif
     if (millis() > _auto_off) {
       _display->turnOff();
     }

examples/companion_radio/ui-orig/UITask.cpp

@@ -342,6 +342,15 @@ void UITask::loop() {
 
       _next_refresh = millis() + 1000;   // refresh every second
     }
+#ifdef KEEP_DISPLAY_ON_USB
+    // Opt-in: refresh the auto-off deadline while externally powered, so the
+    // timer counts from the moment external power is removed. Off by default
+    // because OLED panels burn in quickly; only enable for LCD targets or
+    // where the display is replaceable.
+    if (board.isExternalPowered()) {
+      _auto_off = millis() + AUTO_OFF_MILLIS;
+    }
+#endif
     if (millis() > _auto_off) {
       _display->turnOff();
     }

examples/companion_radio/ui-tiny/UITask.cpp

@@ -704,6 +704,15 @@ void UITask::loop() {
       _display->endFrame();
     }
 #if AUTO_OFF_MILLIS > 0
+#ifdef KEEP_DISPLAY_ON_USB
+    // Opt-in: refresh the auto-off deadline while externally powered, so the
+    // timer counts from the moment external power is removed. Off by default
+    // because OLED panels burn in quickly; only enable for LCD targets or
+    // where the display is replaceable.
+    if (board.isExternalPowered()) {
+      _auto_off = millis() + AUTO_OFF_MILLIS;
+    }
+#endif
     if (millis() > _auto_off) {
       _display->turnOff();
     }