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.cpp

@@ -0,0 +1,100 @@
+#include "BatterySensor.h"
+
+void BatterySensor::begin() {
+  pinMode(PIN_BATTERY_ADC, INPUT);
+  pinMode(PIN_LBO, INPUT);
+
+  analogReadResolution(12);  // 12-bit ADC (0-4095)
+
+  Serial.println("[Battery] Initialized");
+  Serial.println("[Battery] ADC on GPIO 2, LBO on GPIO 5");
+
+  // Initial reading
+  _voltage = readBatteryVoltage();
+  _percent = voltageToPercent(_voltage);
+  _isLow = (_voltage < VOLTAGE_LOW);
+
+  Serial.print("[Battery] Initial voltage: ");
+  Serial.print(_voltage);
+  Serial.print("V, ");
+  Serial.print(_percent);
+  Serial.println("%");
+}
+
+void BatterySensor::loop() {
+  unsigned long now = millis();
+  if (now - _lastCheckMs < CHECK_INTERVAL_MS) return;
+  _lastCheckMs = now;
+
+  float prevVoltage = _voltage;
+  int prevPercent = _percent;
+  bool wasLow = _isLow;
+
+  // Read battery voltage via ADC
+  _voltage = readBatteryVoltage();
+  _percent = voltageToPercent(_voltage);
+
+  // Check LBO pin as backup confirmation
+  bool lboHigh = digitalRead(PIN_LBO);
+  bool lboIndicatesLow = !lboHigh;
+
+  // Battery is low if either voltage is low OR LBO pin indicates low
+  _isLow = (_voltage < VOLTAGE_LOW) || lboIndicatesLow;
+
+  // Log significant changes
+  if (abs(_percent - prevPercent) >= 5 || _isLow != wasLow) {
+    Serial.print("[Battery] Voltage: ");
+    Serial.print(_voltage, 2);
+    Serial.print("V (");
+    Serial.print(_percent);
+    Serial.print("%) LBO: ");
+    Serial.println(lboHigh ? "OK" : "LOW");
+
+    if (_isLow && !wasLow) {
+      Serial.println("[Battery] ⚠️  LOW BATTERY WARNING - Please recharge!");
+    }
+  }
+}
+
+float BatterySensor::readBatteryVoltage() {
+  // Take multiple samples and average them for stability
+  long sum = 0;
+  for (int i = 0; i < SAMPLES; i++) {
+    sum += analogRead(PIN_BATTERY_ADC);
+    delay(5);
+  }
+  int avgRaw = sum / SAMPLES;
+
+  // Convert ADC value to voltage at the pin
+  // ESP32-C3 ADC: 12-bit (0-4095) maps to 0-3.3V
+  float adcVoltage = (avgRaw / 4095.0) * 3.3;
+
+  // Multiply by divider ratio to get actual battery voltage
+  float batteryVoltage = adcVoltage * DIVIDER_RATIO;
+
+  return batteryVoltage;
+}
+
+int BatterySensor::voltageToPercent(float voltage) {
+  // Clamp voltage to valid range
+  if (voltage >= VOLTAGE_MAX) return 100;
+  if (voltage <= VOLTAGE_MIN) return 0;
+
+  // Linear mapping from voltage to percentage
+  // This is a simplified model; LiPo discharge curves are non-linear
+  // For better accuracy, you could use a lookup table
+  float percent = ((voltage - VOLTAGE_MIN) / (VOLTAGE_MAX - VOLTAGE_MIN)) * 100.0;
+
+  return constrain((int)percent, 0, 100);
+}
+
+bool BatterySensor::shouldBlink() const {
+  // Blink every 500ms when battery is low (< 20%)
+  return (_percent < 20) && ((millis() / 500) % 2 == 0);
+}
+
+int BatterySensor::iconFillWidth(int maxWidth) const {
+  // Calculate fill width for battery icon
+  // Returns 0 to maxWidth based on percentage
+  return (_percent * maxWidth) / 100;
+}