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>

README.md

@@ -58,7 +58,9 @@ src/recovery/           ESP32 recovery app: WiFi setup page + OTA
 partitions.csv          ESP32 flash layout (recovery app + two clock firmware slots)
 test/test_nano/         unit tests for the Nano firmware (run on the PC)
 test/test_esp32/        unit tests for the ESP32 logic (run on the PC)
+test/test_driver/       unit tests for the display driver with a simulated panel
 test/mocks/             fake Arduino, EEPROM and display driver for the Nano tests
+test/driver_mocks/      fake Arduino and Adafruit GFX for the driver tests
 tools/                  uploader for flashing the Nano over WiFi, font and picture scripts
 docs/                   documentation (see below)
 Pictures/               photos, pictures of the pages and screenshots of the web UI
@@ -92,7 +94,7 @@ pio run -e nano_ota -t upload    # flash the Nano over WiFi, through the ESP32
 pio run -e esp32 -t upload       # flash the ESP32 over the air
 pio run -e esp32_usb -t upload   # flash the ESP32 over USB
 pio device monitor -e nano       # serial monitor
-pio test -e test_nano -e test_esp32   # run the unit tests on the PC
+pio test -e test_nano -e test_esp32 -e test_driver   # run the unit tests on the PC
 ```
 
 The unit tests need `gcc`/`g++` on your PATH. On Windows you can get it from
@@ -135,3 +137,6 @@ In VS Code you can also pick the environment in the PlatformIO status bar.
 - [ ] **Check the boot timing.** Build the Nano with `MEASURE_REFRESH` to see how long a
   full redraw takes. If the reply wire isn't fitted, check that 3 s
   (`NANO_BOOT_TIMEOUT`) is enough for the Nano to boot.
+- [ ] **Watch the frame check.** After a few days, look at System, "Frame check": any
+  "wrong" or "no answer" counts mean bytes still get lost somewhere; "Nano restarted"
+  means power dips (e.g. while many dots flip).