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/config.h

@@ -55,6 +55,13 @@ const uint32_t HA_ERROR_INTERVAL = 60000;
 // this one (characters per ms at NANO_BAUD) and flipped its changed dots, plus a margin
 const uint32_t NANO_CHARS_PER_MS = NANO_BAUD / 10000;
 const uint32_t FRAME_MARGIN = 20;
+// Frame check (logic/frame_check.h): after a frame the Nano is asked for the checksum of
+// its image (C), at most this often (moving pages send many frames)
+const uint32_t FRAME_CHECK_INTERVAL = 3000;
+// No answer this long: counted as lost (the C or the frame before it got garbled)
+const uint32_t FRAME_CHECK_TIMEOUT = 2000;
+// A wrong image is repaired by sending the whole image again, at most this often
+const uint32_t FRAME_REPAIR_MIN_GAP = 10000;
 
 // ---------------------------------------------------------------------------
 // Flashing the Nano over WiFi (see README.md, pio run -e nano_ota -t upload)
@@ -105,7 +112,7 @@ const uint32_t SETUP_AP_DELAY = 20000;        // open the AP when WiFi fails thi
 const uint32_t WIFI_CONNECT_TIMEOUT = 30000;
 const uint32_t WIFI_RESTART_TIME = 10UL * 60 * 1000;
 
-#define FIRMWARE_VERSION "2.7.2"
+#define FIRMWARE_VERSION "2.8.0"
 
 // Home Assistant device id (do not change: Home Assistant's entities are linked to it)
 const uint8_t HA_DEVICE_ID[] = {0x34, 0x86, 0x5d, 0x3a, 0x08, 0x28};