Fix OLED autosleep timing and battery sampling cadence

- Track last OLED activity to avoid double timeout; keep power gating on transitions
- Copy TZ before setenv() in timegm_fallback to avoid invalid pointer reuse
- Add BATTERY_SAMPLE_INTERVAL_MS and only refresh cache at batch start when due
- Keep battery sampling to a single ADC read (Arduino core lacks explicit ADC power gating)

src/power_manager.cpp

@@ -34,11 +34,8 @@ void power_configure_unused_pins_sender() {
 }
 
 void read_battery(MeterData &data) {
-  uint32_t sum = 0;
-  sum += analogRead(PIN_BAT_ADC);
-  sum += analogRead(PIN_BAT_ADC);
-  float avg = static_cast<float>(sum) / 2.0f;
-  float v = (avg / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
+  uint32_t raw = analogRead(PIN_BAT_ADC);
+  float v = (static_cast<float>(raw) / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
 
   data.battery_voltage_v = v;
   data.battery_percent = battery_percent_from_voltage(v);