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/nano-ota.md

@@ -0,0 +1,74 @@
+# Flashing the Nano over WiFi
+
+[← Back to the README](../README.md)
+
+The ESP32 can flash the Nano for you, so you don't need a USB cable at the display:
+
+```sh
+pio run -e nano_ota -t upload
+```
+
+What happens:
+
+1. The uploader ([tools/nano_net_upload.py](../tools/nano_net_upload.py)) connects to the
+   ESP32 on TCP port **2323** and tells it the bootloader's baud rate.
+2. The ESP32 pauses the clock and switches the relay off for 1 second, then on again. If
+   the reset wire is fitted, it also pulses the Nano's RESET pin. This restarts the Nano,
+   and its bootloader starts.
+3. The ESP32 switches its link to the bootloader's baud rate and passes bytes between
+   WiFi and the Nano.
+4. The uploader writes the new firmware and reads it back to verify it.
+5. When the uploader disconnects, the ESP32 switches the relay off. The clock powers the
+   Nano on again with the new firmware and continues.
+
+The display goes dark for the duration of the upload, which takes a few seconds.
+
+**Requirements**
+
+- The reply wire must be connected: Nano D1 (TX) → voltage divider → ESP32 GPIO 16. The
+  uploader needs the bootloader's answers.
+- The ESP32 must be running firmware with the flash bridge (this version or newer).
+
+## Bootloaders
+
+Flashing the Nano's firmware never changes its bootloader. Both common Nano bootloaders
+are supported:
+
+| Bootloader | Baud | Starts after a power cycle | Reset wire |
+|------------|------|----------------------------|------------|
+| Old bootloader (ATmegaBOOT), the one this Nano has now | 57600 | Yes | Not needed |
+| Optiboot ("new bootloader") | 115200 | No, only after a reset-pin reset | **Required** |
+
+By default (`--baud=auto` in `upload_flags`) the uploader tries 57600 first and then
+115200. Set `--baud=57600` or `--baud=115200` to skip the other attempt.
+
+If you ever replace the bootloader with Optiboot:
+
+- Fit the reset wire: ESP32 GPIO → 1 kΩ → base of an NPN transistor (e.g. 2N2222 or
+  BC547). Connect the collector to Nano RESET and the emitter to GND. Set `NANO_RESET_PIN` in
+  `src/esp32/config.h` to that GPIO, for example `4`. The transistor keeps the Nano's 5 V
+  reset line away from the 3.3 V ESP32 pin.
+- For uploads over USB, change `board` in the `nano` environment to `nanoatmega328new`.
+
+**Settings**
+
+- ESP32 IP address: `upload_port` in the `nano_ota` environment in `platformio.ini`. You
+  can add a port number: `10.1.1.198:2323`. For a one-time different address, use
+  `pio run -e nano_ota -t upload --upload-port 10.1.1.50`.
+- Port, reset pin and timings: `NANO_FLASH_PORT`, `NANO_RESET_PIN`, `NANO_POWER_OFF_TIME`
+  and `NANO_RESET_DELAY` in `src/esp32/config.h`.
+
+**If it fails**
+
+| Message | Cause |
+|---------|-------|
+| `can't connect to ...` | Wrong IP address, or the ESP32 is offline |
+| `the ESP32 flash bridge didn't accept the request` | The ESP32 runs older firmware without the flash bridge. Flash it first (`esp32` or `esp32_usb`). |
+| `no answer from the Nano bootloader` | Reply wire missing, or the Nano has Optiboot and the reset wire is missing |
+| `bad reply to command ...` / `verify failed` | Connection problem. Try again. A failed upload can leave the Nano without working firmware, but the bootloader itself is never overwritten, so flashing again (over WiFi or USB) always fixes it. |
+
+The ESP32 ends the session after 10 seconds without data, so the clock always resumes,
+even if the uploader is interrupted.
+
+> **Security:** anyone on your network can connect to port 2323 and flash the Nano, just
+> like the ESP32's own OTA has no password. Keep the display on a trusted network.