Reduce sender power draw (RX windows + CPU/WiFi/ADC/pins)

- Add LoRa idle/sleep/receive-window helpers and use short RX windows for ACK/time sync

- Schedule sender time-sync windows (fast/slow) and track RX vs sleep time in debug

- Lower sender power (80 MHz CPU, WiFi/BT off, reduced ADC sampling, unused pins pulldown)

- Make SERIAL_DEBUG_MODE a build flag, add prod envs with debug off, and document changes

include/power_manager.h

@@ -5,6 +5,7 @@
 
 void power_sender_init();
 void power_receiver_init();
+void power_configure_unused_pins_sender();
 void read_battery(MeterData &data);
 uint8_t battery_percent_from_voltage(float voltage_v);
 void light_sleep_ms(uint32_t ms);