Add HA dropdowns, off image, flip fixes and refresh effects

Home Assistant: remove the Clock switch (its entity is deleted on connect),
add an IP sensor and Mode / Off image dropdowns whose options follow the
saved bitmaps. Remove the built-in "Nintendo border" bitmap.

Off image: a bitmap (or blank) left on the dots when the display switches off.

Nano: pause after each flip pulse and repeat pulses for changed dots, both
stored in EEPROM (G/N commands). Refresh effects (E command): instant,
sparkle, wipe, rain, from the middle out, diagonal, spiral, flip all then
reveal, push and random, optionally only for small changes. Effects run in
steps from loop() so serial input keeps flowing.

Web UI: flip settings, refresh effect and test patterns under Settings; the
System tab shows why the ESP32 last restarted. Settings records from
versions 1 and 2 are migrated. Firmware 2.3.0.

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

README.md

@@ -95,9 +95,9 @@ 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. |
+| **Bitmaps** | All bitmaps with a preview: show one as the page, send it live, open it in the editor, delete it. The built-in Nintendo logo 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. |
+| **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 pause after each pulse, pulses per dot and the refresh effect (instant, sparkle, wipe, rain, from the middle out, diagonal, spiral, flip all then reveal, push, random; optionally only for small changes like the clock's minutes); test patterns; 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. |
 
@@ -293,10 +293,15 @@ The ESP32 talks to the Nano on **UART2** (`Serial2`, 115200 baud). The ESP32's o
      shown from 17:00.
    - **22:00–23:30:** the clock is only shown during the minutes :00, :15, :30 and :45.
      30 seconds after each of those minutes, the relay turns the display off.
-   - **23:31–08:00:** the screen is cleared and power is cut after 30 seconds.
-3. **Home Assistant switches** (MQTT discovery):
-   - `FlipDot Display Clock`: off shows a Nintendo logo instead of the clock.
-   - `FlipDot Display Power`: off clears the display and cuts power through the relay.
+   - **23:31–08:00:** the screen is cleared (or shows the off image chosen in the web UI)
+     and power is cut after 30 seconds.
+3. **Home Assistant** (MQTT discovery):
+   - `FlipDot Display Power` switch: off clears the display and cuts power through the relay.
+   - `FlipDot Display Mode` dropdown: what is shown during the day, the clock 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.
 4. **Nano.** The Nano reads commands from the serial line and updates its frame buffer.
    On `D` it flips only the dots that changed. Every 2 minutes it redraws the whole screen
    to fix dots that did not flip.
@@ -324,6 +329,10 @@ The serial protocol is described in [src/nano/README.md](src/nano/README.md).
 - [X] **Test flashing over WiFi on the real hardware.** The uploader is tested against a
   simulated bootloader, not yet against the real Nano. Check especially that the old
   bootloader starts after the relay power cycle.
+- [ ] **Tune the flip settings on the real hardware.** Flash both the Nano and the ESP32
+  (the old Nano ignores the new `G`/`N`/`E` commands). With the test patterns (Settings),
+  try flip gap 0/200/500 µs and 1 or 2 pulses, and check that the refresh effects look
+  right and that the off image is complete before the power is cut.
 - [ ] **Check the boot timing.** Build the Nano with `MEASURE_REFRESH` to see how long a
   full redraw takes. If the reply wire isn't fitted, check that 3 s
   (`NANO_BOOT_TIMEOUT`) is enough for the Nano to boot.