Add Battery Sensor functionality and integrate with display and app

- Implement BatterySensor class to monitor battery voltage and status.
- Update App to initialize and loop BatterySensor.
- Modify FaceRenderer to display battery status on the screen.
- Enhance Display class with a method to draw battery icon.
- Update WebUI to include battery status in the interface.
- Refactor WiFiManager to improve connection handling and logging.
- Adjust Settings to include factory reset functionality.
- Improve HTML structure and styling in WebUI for better user experience.

src/sensors/BatterySensor.h

@@ -0,0 +1,40 @@
+#pragma once
+#include <Arduino.h>
+
+class BatterySensor {
+public:
+  void begin();
+  void loop();
+
+  // Battery status
+  int percent() const { return _percent; }
+  float voltage() const { return _voltage; }
+  bool isLow() const { return _isLow; }
+  bool shouldBlink() const;
+
+  // For display
+  int iconFillWidth(int maxWidth) const;
+
+private:
+  static constexpr int PIN_BATTERY_ADC = 2;  // Voltage divider input
+  static constexpr int PIN_LBO = 5;          // PowerBoost LBO pin
+
+  static constexpr unsigned long CHECK_INTERVAL_MS = 2000;
+  static constexpr int SAMPLES = 10;  // Number of ADC samples to average
+
+  // LiPo voltage thresholds
+  static constexpr float VOLTAGE_MIN = 3.0;   // 0%
+  static constexpr float VOLTAGE_MAX = 4.2;   // 100%
+  static constexpr float VOLTAGE_LOW = 3.2;   // Low battery threshold
+
+  // Voltage divider: R1=100k, R2=100k (2:1 ratio)
+  static constexpr float DIVIDER_RATIO = 2.0;
+
+  int _percent = 100;
+  float _voltage = 3.7;
+  bool _isLow = false;
+  unsigned long _lastCheckMs = 0;
+
+  float readBatteryVoltage();
+  int voltageToPercent(float voltage);
+};