Keep receiver LoRa in continuous RX

- Add lora_receive_continuous() helper and use it after init and TX (ACK/time sync)

- Ensure receiver returns to RX immediately after lora_send

- Document continuous RX behavior in README

src/main.cpp

@@ -431,6 +431,7 @@ static void send_batch_ack(uint16_t batch_id, uint16_t sender_id) {
   write_u16_le(&ack.payload[2], sender_id);
   write_u16_le(&ack.payload[4], g_short_id);
   lora_send(ack);
+  lora_receive_continuous();
 }
 
 static bool prepare_inflight_from_queue() {
@@ -651,6 +652,7 @@ void setup() {
     update_battery_cache();
   } else {
     power_receiver_init();
+    lora_receive_continuous();
     pinMode(PIN_ROLE, INPUT); // release pulldown before SD uses GPIO14 as SCK
     sd_logger_init();
     wifi_manager_init();