Fix shifted pictures: non-blocking Nano drawing, frame check, refresh settings

The Nano's periodic full redraw blocked for ~0.8 s. A frame that arrived
meanwhile overflowed its 64-byte serial buffer; lost bytes in a bitmap
shift the rest of the image (picture moved up), and diff frames never
repaired it.

- Nano: all drawing (frames, effects, the dot refresh) runs from update()
  in ~1 ms slices; the refresh pauses while a frame is received or shown
- New commands: C (reply with the image checksum) and I (refresh minutes,
  EEPROM)
- ESP32: NanoMonitor asks for the checksum after frames and sends the whole
  image again on a mismatch, missing answer or Nano restart; counts ERR
  replies. Periodic whole-image resync.
- Settings: "Dot refresh" and "Image resync" intervals (settings v5,
  v4 records are migrated)
- System tab: loop timing, free memory, frame check counts
- test_driver: the real driver against a simulated panel

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/main.cpp

@@ -73,6 +73,7 @@ void loop()
 	// OTA and the firmware guard come first, so they keep working whatever happens below
 	otaLoop();
 	firmwareGuardLoop(wifiConnected());
+	loopStats.tick(micros());
 	wifiWatchdog();
 
 	homeAssistantSetFirmwareStatus(firmwareGuardStatus());
@@ -92,6 +93,10 @@ void loop()
 	bool time_valid = appLocalTime(now);
 	appPageLoop(time_valid, now);
 	controller.update(millis(), time_valid, now, wifiConnected());
+	// Checks the frames just sent; a wrong image on the Nano (and every resync interval):
+	// the whole image again with the next update
+	if (nanoMonitor.loop(millis(), appSettings().refresh.image * 60000UL))
+		controller.pageChanged();
 
 	bool led = appSettings().status_led && statusLedLevel(millis(), wifiConnected(), nano.isOn());
 	digitalWrite(LED_PIN, led);