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/home-assistant.md

@@ -0,0 +1,49 @@
+# Home Assistant
+
+[← Back to the README](../README.md)
+
+The ESP32 connects to Home Assistant in two ways:
+
+- **MQTT** (with discovery): the display's own switches, dropdowns, sensors and buttons.
+  Broker, user and password are in the web UI, Home Assistant tab (an empty broker uses
+  `secrets.h`). Changing them restarts the ESP32.
+- **REST API**: values of any entity for the dynamic pages and the trigger rules. Set the
+  URL (empty: `http://<broker>:8123`, plain http only, use the IP address) and a
+  long-lived access token (Home Assistant, your profile, Security tab) in the same tab.
+
+## Entities
+
+- `FlipDot Display Power` switch: off clears the display and cuts power through the relay.
+- `FlipDot Display Mode` dropdown: what is shown during the day, a dynamic page ("Clock"
+  first) or one of the bitmaps (the same choice as in the web UI).
+- `FlipDot Display Off image` dropdown: what is left on the dots when the display
+  switches off (at night or with the Power switch): blank or one of the bitmaps.
+- `FlipDot Display IP` sensor: the ESP32's IP address.
+- `FlipDot Display Stopwatch` switch and `FlipDot Display Stopwatch reset` button: the
+  stopwatch of the dynamic pages.
+- `FlipDot Display Shown page` sensor: what the display shows now (schedule, rule or
+  chosen page). `FlipDot Display Back to schedule` button: ends a page chosen by hand.
+- `FlipDot Display Firmware` sensor: version and health, e.g. `2.7.2, ok, 0 crashes, recovery ok`.
+- `FlipDot Display Recovery` button: restarts into the [recovery app](recovery.md) for 10
+  minutes.
+
+The unique ids of these entities never change, so automations and dashboards keep working
+after firmware updates.
+
+## Values on dynamic pages
+
+A Home Assistant value element on a [page](pages.md) shows an entity's state or one of
+its attributes, as a number (multiplier, decimals, auto unit, without minus sign), a
+duration, a time from a timestamp, or with own texts per state (see the
+[web UI](web-ui.md#pages)).
+
+**Home Assistant values** on the shown page are read every minute (one entity per loop,
+at most about 3 s). An unknown or unreachable value shows `--`. The display only changes
+when the shown text changes, so round values with the decimals setting.
+
+## Rules
+
+The [schedule](web-ui.md#schedule) can bring a page to the front from a Home Assistant
+state, e.g. the 3D printer page while `sensor.mainsailos_current_print_state` is
+`printing`. Each rule is read at its own interval; when Home Assistant can't be reached it
+is read once a minute at most, so the display keeps running.