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/app.h

@@ -8,16 +8,20 @@
 #include "display/status_screen.h"
 #include "logic/display_controller.h"
 #include "logic/local_time.h"
+#include "logic/loop_stats.h"
 #include "logic/page.h"
 #include "logic/page_schedule.h"
 #include "logic/settings.h"
 #include "nano_link/nano_link.h"
+#include "nano_link/nano_monitor.h"
 
 extern NanoLink nano;
 extern BROSE9323 display;
 extern FlipdotFrames frames;
 extern StatusScreen status;
 extern DisplayController controller;
+extern LoopStats loopStats;
+extern NanoMonitor nanoMonitor;
 
 // Settings: loaded at boot, changed in the web UI
 const Settings &appSettings();