feat(battery): round bat_v and derive bat_pct locally

src/power_manager.cpp

@@ -32,14 +32,18 @@ void read_battery(MeterData &data) {
   float v = (avg / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
 
   data.battery_voltage_v = v;
-  float pct = (v - 3.0f) / (4.2f - 3.0f) * 100.0f;
+  data.battery_percent = battery_percent_from_voltage(v);
+}
+
+uint8_t battery_percent_from_voltage(float voltage_v) {
+  float pct = (voltage_v - 3.0f) / (4.2f - 3.0f) * 100.0f;
   if (pct < 0.0f) {
     pct = 0.0f;
   }
   if (pct > 100.0f) {
     pct = 100.0f;
   }
-  data.battery_percent = static_cast<uint8_t>(pct + 0.5f);
+  return static_cast<uint8_t>(pct + 0.5f);
 }
 
 void go_to_deep_sleep(uint32_t seconds) {