Keep sender OLED on longer; faster page cycle

src/main.cpp

@@ -120,7 +120,11 @@ static void sender_cycle() {
     time_handle_timesync_payload(rx.payload, rx.payload_len);
   }
 
-  delay(50);
+  uint32_t start = millis();
+  while (millis() - start < SENDER_OLED_READ_MS) {
+    display_tick();
+    delay(50);
+  }
   display_power_down();
   lora_sleep();
   go_to_deep_sleep(SENDER_WAKE_INTERVAL_SEC);