Add battery voltage reading to web UI, apply fixed VBAT multiplier, closes #3

source/battery.cpp

@@ -20,8 +20,10 @@ void initBatteryADC() {
 float readBatteryVoltage() {
   // 8 quick reads, no delay() calls, no analogReference() change
   int32_t raw=0; for (int i=0; i<8; i++) raw += analogRead(PIN_VBAT_READ); raw /= 8;
-  // Seeed XIAO nRF52840 Sense: 1MΩ + 510kΩ voltage divider on VBAT → multiply by 1510/510
-  return (raw / 4096.0f) * 3.0f * (1510.0f / 510.0f);
+  // Seeed XIAO nRF52840 Sense: 1MΩ + 510kΩ voltage divider on VBAT.
+  // Theoretical ratio is 1510/510 = 2.961, but real resistor tolerances
+  // shift the actual ratio.  Calibrated: 3.90V actual / 3.78V reported → ×1.0317.
+  return (raw / 4096.0f) * 3.0f * (1510.0f / 510.0f) * 1.0317f;
 }
 
 int batteryPercent(float v) {