feat: Add OLED size build flag support and implement PowerManager for enhanced power management

src/power/PowerManager.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <Arduino.h>
+#include "../sensors/BatterySensor.h"
+#include "../ui/Display.h"
+
+class PowerManager {
+public:
+  void begin(unsigned long bootMs);
+  void setBatteryAwakeWindowMs(unsigned long windowMs);
+  void loop(const BatterySensor& battery);
+
+  bool isPluggedIn() const { return _pluggedIn; }
+  bool batteryWindowElapsed(unsigned long nowMs) const;
+
+  void deepSleepForSeconds(uint32_t seconds, const char* reason, Display* display = nullptr) const;
+
+private:
+  bool detectPluggedIn(const BatterySensor& battery) const;
+
+  bool _pluggedIn = true;
+  bool _plugStateInitialized = false;
+  unsigned long _bootMs = 0;
+  unsigned long _onBatterySinceMs = 0;
+  unsigned long _batteryAwakeWindowMs = 2UL * 60UL * 1000UL;
+};