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/logic/settings.h

@@ -54,6 +54,18 @@ struct DriveSettings
 	bool effect_small_only = false; // big changes (a new page) appear instantly
 };
 
+// Full redraws that fix the picture, in minutes (0 = never)
+const uint16_t REFRESH_MINUTES_MAX = 1440;
+struct RefreshSettings
+{
+	// The Nano flips every dot once more (fixes dots that didn't flip). Sent to the Nano,
+	// which keeps it in its EEPROM (I command, default there: 2).
+	uint16_t dots = 2;
+	// The ESP32 sends the whole image again (fixes a wrong image on the Nano, e.g. after
+	// lost bytes). Only the dots that differ flip, so a correct image doesn't change.
+	uint16_t image = 10;
+};
+
 // What is shown during the day: the name of a dynamic page (logic/page.h) or a bitmap.
 // Both share one namespace. When it is gone, the built-in Clock page is shown.
 #define DEFAULT_PAGE BUILTIN_PAGE_CLOCK
@@ -70,6 +82,7 @@ struct Settings
 	bool status_led = true;
 	char off_page[PAGE_NAME_SIZE] = OFF_PAGE_BLANK;
 	DriveSettings drive;
+	RefreshSettings refresh;
 	// New fields go here, at the end (see system/settings_store.cpp)
 };