feat: power manager with battery monitoring, LED warnings, and sleep modes

Co-authored-by: Cursor <cursoragent@cursor.com>

src/power_manager.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <Arduino.h>
+#include "config.h"
+
+class PowerManager {
+public:
+    void begin();
+    uint8_t getBatteryPercent();
+    bool isCharging();
+    void updateBatteryLED();
+    void flashLED(uint8_t r, uint8_t g, uint8_t b, uint16_t duration_ms = LED_PULSE_DURATION_MS);
+    void enterDeepSleep();
+    void enableLightSleep();
+    void disableEPDPower();
+    void enableEPDPower();
+
+private:
+    uint8_t _lastBatteryPct = 100;
+    unsigned long _lastLEDPulse = 0;
+    uint8_t _ledBrightness = DEFAULT_LED_BRIGHTNESS;
+
+    void setLED(uint8_t index, uint8_t r, uint8_t g, uint8_t b);
+    void clearLEDs();
+};