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

src/lora_transport.cpp

@@ -110,10 +110,24 @@ bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms) {
   }
 }
 
+void lora_idle() {
+  LoRa.idle();
+}
+
 void lora_sleep() {
   LoRa.sleep();
 }
 
+bool lora_receive_window(LoraPacket &pkt, uint32_t timeout_ms) {
+  if (timeout_ms == 0) {
+    return false;
+  }
+  LoRa.receive();
+  bool got = lora_receive(pkt, timeout_ms);
+  LoRa.sleep();
+  return got;
+}
+
 uint32_t lora_airtime_ms(size_t packet_len) {
   if (packet_len == 0) {
     return 0;