Fix dense images on the Nano and the off image in slow mode

- Nano: B data is drawn fast enough for the serial line. A hex digit arrives
  every 87 us; with a division and four drawPixel() calls per digit a lit
  digit took about 105 us, so dense frames (all yellow, the panda bitmap)
  overflowed the 64 byte receive buffer: frames not shown or wrong, "Line
  too long", frame check repairs that failed the same way. Now the parser
  counts columns and rows and BROSE9323::drawDots() sets the 4 dots
  (about 20 us).
- ESP32: Power off and the start of the night draw the off image also when
  slow mode had the power off already (it powers on for it, once)
- Tests: drawDots() against drawPixel(), off image in slow mode
- Version 2.19.3

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/how-it-works.md

@@ -29,7 +29,8 @@
      Schedule pages and rotation rules with "skip in slow mode" are left out of the rotation
      (none left in the block: the chosen page).
    - **23:31–08:00:** the screen is cleared (or shows the off image chosen in the web UI)
-     and power is cut after 30 seconds.
+     and power is cut after 30 seconds. When slow mode had the power off already, it is
+     switched on for this. The same goes for the Power switch.
 3. **Home Assistant**: switches, dropdowns and sensors over MQTT, see
    [Home Assistant](home-assistant.md).
 4. **Nano.** The Nano reads commands from the serial line and updates its frame buffer.