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/nano_link/nano_monitor.cpp

@@ -0,0 +1,43 @@
+#include "nano_link/nano_monitor.h"
+
+NanoMonitor::NanoMonitor(NanoLink &nano, BROSE9323 &display) : _nano(nano), _display(display)
+{
+}
+
+bool NanoMonitor::loop(uint32_t now, uint32_t resync_ms)
+{
+	bool on = _nano.isOn();
+	if (on != _was_on)
+	{
+		// Switched on (the Nano booted, NanoLink::on() took its READY) or off
+		_was_on = on;
+		_check.reset(now);
+		_len = 0;
+	}
+	if (!on)
+		return false;
+
+	// First the answers that arrived (also while loop() was blocked), then the timeouts
+	Stream &serial = _nano.stream();
+	while (serial.available())
+	{
+		char c = serial.read();
+		if (c == '\n')
+		{
+			_line[_len] = '\0';
+			_check.line(_line, now);
+			_len = 0;
+		}
+		else if (_len < sizeof(_line) - 1)
+			_line[_len++] = c;
+	}
+	bool resync = _check.resyncNow(now, resync_ms);
+
+	uint32_t frames = _display.framesSent();
+	if (_check.due(frames, now))
+	{
+		serial.print("C\n");
+		_check.sent(_display.crc(), frames, now);
+	}
+	return resync;
+}