Split the README into docs/ pages, add web UI screenshots

- README is the overview; web UI, pages, Home Assistant, how it works,
  wiring, Nano over WiFi and the recovery app have their own page in docs/.
- docs/web-ui.md has a screenshot of every tab, made with headless Edge by
  tools/make_webui_screenshots.py.
- The web UI opens a page in the editor with #pages/<name>. Version 2.7.2.
- The reply wire is fitted (flashing the Nano over WiFi works).

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

docs/how-it-works.md

@@ -0,0 +1,26 @@
+# How it works
+
+[← Back to the README](../README.md)
+
+1. **ESP32 boot.** The ESP32 connects to WiFi and starts ArduinoOTA. After a crash it
+   runs only OTA for 15 seconds, and after repeated crashes it goes to safe mode. Bad OTA
+   firmware is rolled back automatically, see
+   [Keeping OTA alive](../src/esp32/README.md#keeping-ota-alive). It then switches the display relay on, waits for
+   the Nano's `READY` message (at most 3 s), shows "Connecting...", starts NTP and
+   connects to MQTT.
+2. **Clock.** Once the time is synced, the ESP32 sends a new frame whenever the screen
+   changes, mostly once a minute:
+   - **08:00–22:00:** the page: the chosen page (by default the built-in Clock page,
+     Pikachu with a thought bubble showing `HH:MM`, yawning from 17:00), or what the
+     [schedule and rules](web-ui.md#schedule) pick.
+   - **22:00–23:30:** the clock is only shown during the minutes :00, :15, :30 and :45.
+     30 seconds after each of those minutes, the relay turns the display off.
+   - **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.
+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.
+   On `D` it flips only the dots that changed. Every 2 minutes it redraws the whole screen
+   to fix dots that did not flip.
+
+The serial protocol is described in [src/nano/README.md](../src/nano/README.md).