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/recovery.md

@@ -0,0 +1,82 @@
+# Recovery app
+
+[← Back to the README](../README.md)
+
+The ESP32 has a small second app, the **recovery app** ([src/recovery](../src/recovery)). It
+has its own place on the flash and is never changed by OTA updates. It contains only WiFi,
+a setup web page and OTA, so the ESP32 can always be reached, even when the clock firmware
+is broken. It shows what is going on as a short message on the display, and the built-in
+LED double-blinks.
+
+```
+flash (4 MB):  bootloader | nvs | otadata | factory: recovery | ota_0: clock | ota_1: clock | bitmaps
+```
+
+**When it starts**
+
+| Trigger | Back to the clock |
+|---------|-------------------|
+| The clock firmware can't connect to WiFi (30 s at boot, or 10 min later on) | As soon as the WiFi works again (after at least 1 minute) |
+| The clock firmware crashed 3 times in a row | After 10 minutes |
+| **FlipDot Display Recovery** button in Home Assistant | After 10 minutes |
+| Switching the power on and off 3 times, each time within 10 s | After 10 minutes |
+| There is no working clock firmware (the bootloader falls back to it) | Never by itself |
+
+The setup page can also start the clock right away, or keep the recovery app running
+("Stay in recovery").
+
+**What the display shows**
+
+| Message | Meaning |
+|---------|---------|
+| `Connecting` / `to WiFi...` | The clock firmware is starting |
+| `Getting` / `the time...` | Connected, waiting for the time from the internet (NTP) |
+| `No WiFi` / `12:34` | The clock firmware lost WiFi for more than a minute; the time keeps running |
+| `Crashed!` / `OTA open 15s` | The clock firmware crashed and waits 15 s for an OTA update before it starts again |
+| `Crashed 3x` / `OTA only` | Repeated crashes without the recovery app installed: only OTA for 10 minutes |
+| `Recovery` / reason | The recovery app started (`no WiFi`, `crashed`, `requested`, `power cycled`, `no firmware`) |
+| `WiFi setup:` / `FlipDot-Setup` | Connect to this WiFi to choose another network, see below |
+| `Recovery` / IP address | The recovery app is connected; open this address for the setup page |
+
+The recovery app switches the display off after each message; the dots keep showing it
+without power.
+
+**WiFi setup.** The recovery app keeps trying the stored WiFi. If that fails for 20 s, it
+also opens its own WiFi: **FlipDot-Setup**, with the password `SETUP_AP_PASSWORD` from
+`src/esp32/secrets.h`. Connect a phone to it; the setup page opens by itself (otherwise go
+to <http://192.168.4.1>). Pick a network, enter the password and save. The new WiFi is
+stored in the ESP32's NVS and used by both apps from then on (instead of `WIFI_SSID` from
+`secrets.h`). The setup page is also reachable on your normal WiFi, at the ESP32's IP
+address.
+
+**Updating the clock firmware from the recovery app.** Upload as usual,
+`pio run -e esp32 -t upload`. On the setup WiFi, add `--upload-port 192.168.4.1`. The
+recovery app writes the firmware to `ota_0` and starts it.
+
+**Updating the recovery app itself** needs USB (`pio run -e recovery -t upload`). It
+should rarely be necessary.
+
+## One-time installation (USB)
+
+This changes the flash layout, so it can only be done over USB. It is the last USB flash
+the ESP32 needs. Best done together with the wiring TODOs, while the display is open
+anyway.
+
+1. Connect the ESP32 over USB (`COM11`, see `platformio.ini`).
+2. Erase the flash and install the recovery app:
+   ```sh
+   pio run -e recovery -t erase
+   pio run -e recovery -t upload
+   ```
+   The recovery app starts (LED double-blinks, the display shows `Recovery` /
+   `no firmware`), connects to the WiFi from `secrets.h` and shows its IP address. It waits
+   there, because there is no clock firmware yet.
+3. Install the clock firmware **over WiFi**. This also tests that the recovery app works:
+   ```sh
+   pio run -e esp32 -t upload
+   ```
+4. Check Home Assistant: the **FlipDot Display Firmware** sensor should show
+   `2.1.0, testing, ...`, and after a minute `2.1.0, ok, 0 crashes, recovery ok`.
+
+Erasing the flash also clears the NVS: WiFi settings made on the setup page are reset to
+the ones in `secrets.h`.