Add configurable battery and USB display behavior

Persist separate display modes and timeouts in seconds for battery and USB power, expose four controls in WebConfig and CLI, and apply one shared policy across display-equipped roles.

Detect R8 external power from a USB host or calibrated battery voltage above 4.21 V, with cached sampling and hysteresis. Add policy, persistence, browser, and R8 power-detection regression coverage.

examples/simple_repeater/main.cpp

@@ -96,10 +96,7 @@ void setup() {
 #ifdef DISPLAY_CLASS
   display_ready = display.begin();
   if (display_ready) {
-    display.startFrame();
-    display.setCursor(0, 0);
-    display.print("Please wait...");
-    display.endFrame();
+    display.turnOff();  // Stay dark until the saved display policy is loaded.
   }
 #endif