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/time_manager.cpp

@@ -83,7 +83,11 @@ bool time_send_timesync(uint16_t device_id_short) {
   pkt.payload_type = PayloadType::TimeSync;
   pkt.payload_len = compressed_len;
   memcpy(pkt.payload, compressed, compressed_len);
-  return lora_send(pkt);
+  bool ok = lora_send(pkt);
+  if (ok) {
+    lora_receive_continuous();
+  }
+  return ok;
 }
 
 bool time_handle_timesync_payload(const uint8_t *payload, size_t len) {