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/wiring.md

@@ -0,0 +1,14 @@
+# Wiring between ESP32 and Nano
+
+[← Back to the README](../README.md)
+
+The ESP32 talks to the Nano on **UART2** (`Serial2`, 115200 baud). The ESP32's own
+`Serial` (UART0, USB) is free for debug output.
+
+| ESP32 | Nano | Notes |
+|-------|------|-------|
+| GPIO 17 (TX2) | D0 (RX) | Display commands |
+| GPIO 16 (RX2) | D1 (TX) | `READY` and `ERR` replies, and bootloader answers when flashing over WiFi. Use a 5 V → 3.3 V voltage divider (e.g. 1 kΩ / 2 kΩ), because the ESP32 is not 5 V tolerant. Without it, the ESP waits a fixed 3 s after power-on, and `nano_ota` doesn't work. |
+| GPIO set in `NANO_RESET_PIN` | RESET (through an NPN transistor) | Optional, only needed to flash a Nano with Optiboot over WiFi. See [Bootloaders](nano-ota.md#bootloaders). |
+| GND | GND | Common ground |
+| GPIO 13 | Relay input | Relay powers the panels and the Nano (`LOW` = on) |