Add a web UI to the clock firmware

- Web UI on port 80: live view of the display, choose the page (clock or
  a bitmap), bitmaps (list, show, live, edit, delete), draw / image upload
  with conversion in the browser, live mode, settings, Home Assistant
- Live view from a shadow buffer in the BROSE9323 driver (shownPixel)
- Settings in NVS with the original behaviour as defaults: schedule times,
  quarter hours, yawning Pikachu, No WiFi screen, flip time, status LED,
  Home Assistant broker (restart on change)
- Bitmaps on LittleFS; new 320 KB partition, ota slots now 1.31 MB
- DisplayController: pages, live mode, settings; app.cpp holds the objects
- README for the recovery app; docs for the web UI
- Tests: settings, time parsing, bitmap format and names, schedule with
  settings, pages and live mode (95 total)

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

README.md

@@ -23,7 +23,8 @@ too, and it boots again each time the relay switches on. Only the ESP32 runs all
 | Part | Folder | What it does |
 |------|--------|--------------|
 | **Arduino Nano** | [src/nano](src/nano/README.md) | Drives the panel's row and column electronics directly. Keeps a frame buffer and flips only the dots that changed. Takes simple text commands over serial. |
-| **ESP32** | [src/esp32](src/esp32/README.md) | Connects to WiFi, gets the time over NTP, shows a clock with Pikachu bitmaps, exposes switches to Home Assistant over MQTT, switches display power with a relay and supports OTA updates. |
+| **ESP32** | [src/esp32](src/esp32/README.md) | Connects to WiFi, gets the time over NTP, shows a clock with Pikachu bitmaps or your own bitmaps, has a [web UI](#web-ui), exposes switches to Home Assistant over MQTT, switches display power with a relay and supports OTA updates. |
+| **ESP32 recovery app** | [src/recovery](src/recovery/README.md) | Small second app: WiFi setup page and OTA, so the ESP32 stays reachable when the clock firmware fails. |
 | **BROSE9323 driver** | [lib/BROSE9323](lib/BROSE9323/README.md) | Shared `Adafruit_GFX` display driver. On the Nano it toggles GPIO pins. On the ESP32 (`-D ESPSERIAL`) it turns every drawing call into a serial command for the Nano. |
 
 Because the driver is an `Adafruit_GFX` subclass on both boards, the ESP32 can use the
@@ -38,6 +39,7 @@ lib/BROSE9323/          shared flipdot driver (used by both targets)
 src/nano/               Arduino Nano firmware
 src/esp32/              ESP32 clock firmware
   secrets.example.h     template for WiFi / MQTT credentials
+  web/                  the web UI
 src/recovery/           ESP32 recovery app: WiFi setup page + OTA
 partitions.csv          ESP32 flash layout (recovery app + two clock firmware slots)
 test/test_nano/         unit tests for the Nano firmware (run on the PC)
@@ -85,6 +87,31 @@ In VS Code you can also pick the environment in the PlatformIO status bar.
 > also uses. Disconnect that wire before you flash the Nano over USB, or the upload may
 > fail. Flashing over WiFi (`nano_ota`) doesn't have this problem.
 
+## Web UI
+
+Open `http://<ESP32 IP>/` (e.g. <http://10.1.1.198/>) in a browser. At the top is a live
+view of the display: it shows what the dots show right now, updated every 1.5 s.
+
+| Tab | What you can do |
+|-----|-----------------|
+| **Show** | Choose what the display shows during the day: the clock (Pikachu with the time) or any bitmap. The schedule still decides when the display is on. |
+| **Bitmaps** | All bitmaps with a preview: show one as the page, send it live, open it in the editor, delete it. Two built-in ones (Nintendo) can't be deleted. |
+| **Draw / Live** | Draw on an 84 x 19 canvas (click or drag), or load an image: it is converted in the browser (threshold, invert, dithering, fit / fill / stretch). "Show on display now" sends it to the display right away (*live mode*); with "live while drawing" every change appears on the dots. Save it as a bitmap with a name. |
+| **Settings** | Schedule on/off, when the display is on, the late-evening quarter-hour mode, when it switches off; yawning Pikachu and its time; the "No WiFi" screen; the flip time; the status LED. |
+| **Home Assistant** | On/off, broker, port, user, password. Saving restarts the ESP32. An empty broker uses `secrets.h`. |
+| **System** | Firmware status, uptime, WiFi, storage; restart; start the recovery app. |
+
+**Live mode** shows the image at once, also at night or before the time is known. It
+ends with "Back to normal", or 30 minutes after the last image. The Home Assistant Power
+switch still wins: when it is off, nothing is shown.
+
+**Bitmaps** are stored on the ESP32 (LittleFS, 320 KB, room for well over a thousand). They
+need the new flash layout from the [one-time installation](#one-time-installation-usb);
+before that, only the built-in ones are available.
+
+Settings are stored in the ESP32's NVS. The defaults are the original behaviour. There is
+no login: anyone on your WiFi can use the page, as with OTA.
+
 ## Flashing the Nano over WiFi
 
 The ESP32 can flash the Nano for you, so you don't need a USB cable at the display:
@@ -167,7 +194,7 @@ is broken. It shows what is going on as a short message on the display, and the
 LED double-blinks.
 
 ```
-flash (4 MB):  bootloader | nvs | otadata | factory: recovery app | ota_0: clock | ota_1: clock
+flash (4 MB):  bootloader | nvs | otadata | factory: recovery | ota_0: clock | ota_1: clock | bitmaps
 ```
 
 **When it starts**
@@ -282,6 +309,8 @@ The serial protocol is described in [src/nano/README.md](src/nano/README.md).
   [One-time installation](#one-time-installation-usb)). Until then the ESP32 runs the old
   flash layout: the clock firmware works, but without the recovery app; the Home
   Assistant sensor shows "no recovery app".
+- [ ] **Test the web UI on the real hardware**: live view, uploading and drawing a bitmap,
+  live mode, choosing a page, settings, Home Assistant settings (restart).
 - [ ] **Test the recovery app on the real hardware**: the Home Assistant button, the setup
   WiFi with a wrong WiFi password, 3 quick power cycles, and an OTA upload into it.