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>

test/mocks/BROSE9323.h

@@ -19,6 +19,8 @@ public:
 	std::vector<DisplayCall> calls;
 	bool direct = false;
 	bool effect_running = false; // what busy() returns
+	bool refresh_running = false; // what refreshing() returns
+	uint16_t crc_value = 0;       // what crc() returns
 
 	BROSE9323(uint8_t, uint8_t, uint8_t, uint16_t) {}
 
@@ -33,6 +35,9 @@ public:
 	void setEffect(uint8_t effect, uint16_t time_ms, uint8_t flags = 0) { log("effect", effect, time_ms, flags); }
 	bool update() { return effect_running; }
 	bool busy() const { return effect_running; }
+	void startRefresh() { log("refresh"); }
+	bool refreshing() const { return refresh_running; }
+	uint16_t crc() const { return crc_value; }
 	void printBuffer() { log("printBuffer"); }
 	void setCursor(int16_t x, int16_t y) { log("cursor", x, y); }
 	void setTextColor(uint16_t color, uint16_t background) { log("textColor", color, background); }