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>

src/esp32/README.md

@@ -29,7 +29,7 @@ debug output. Wiring is described in the [main README](../../README.md#wiring-be
 | `system/system_store.*` | Settings in NVS: WiFi, recovery reason, firmware slot and version, power-on count |
 | `system/settings_store.*` | The web UI settings in NVS |
 | **`display/`** | **What is drawn** |
-| `display/frames.*` | The screens: clock, "No WiFi", page bitmap, live image, logo, blank (`FlipdotFrames`) |
+| `display/frames.*` | The screens: clock, "No WiFi", page bitmap, live image, off image (`FlipdotFrames`) |
 | `display/bitmap_store.*` | Built-in bitmaps and the uploaded ones on LittleFS |
 | `display/status_screen.*` | Two-line status messages at boot, after a crash and in the recovery app (`StatusScreen`) |
 | `display/text.*` | Centered text lines on the 84 x 19 display |
@@ -39,7 +39,7 @@ debug output. Wiring is described in the [main README](../../README.md#wiring-be
 | `nano_link/nano_flash_bridge.*` | WiFi to serial bridge for flashing the Nano over WiFi (TCP port 2323) |
 | **`net/`** | **Network** |
 | `net/wifi_ota.*` | WiFi (with a restart when it stays down), ESP32 OTA updates, OTA-only mode |
-| `net/home_assistant.*` | MQTT and the Home Assistant entities (two switches, firmware sensor, recovery button) |
+| `net/home_assistant.*` | MQTT and the Home Assistant entities (power switch, mode and off image dropdowns, IP and firmware sensors, recovery button) |
 | **`web/`** | **Web UI** (see the [main README](../../README.md#web-ui)) |
 | `web/web_ui.*` | Web server and JSON API (the endpoints are listed in `web_ui.h`) |
 | `web/web_page.h` | The page: HTML and JavaScript; the browser converts images and draws the previews |
@@ -90,20 +90,25 @@ Rules for changing the code:
   [main README](../../README.md#recovery-app).
 
 - **Frames only when something changes:** a new frame is sent when the minute changes, the
-  screen changes (clock, logo, blank) or the Nano was just powered on. The serial line is
+  screen changes (clock, page, off image) or the Nano was just powered on. The serial line is
   quiet in between, which also gives the Nano time for its periodic full redraw.
 - **OTA updates** through ArduinoOTA. Hostname: `FlipDotDisplay`.
 - **Robust OTA:** rollback, watchdog, OTA window after a crash and safe mode, see
   [Keeping OTA alive](#keeping-ota-alive).
 - **NTP time** with timezone `CET-1CEST,M3.5.0,M10.5.0/3` (Europe/Amsterdam). Until the
   time is synced, the display shows "Connecting...".
-- **Home Assistant** through MQTT (`home-assistant-integration` library) with two switches.
-  Each switch reports its new state back, so Home Assistant always shows the real state.
-  - `FlipDot Display Clock`: on = clock; off = Nintendo logo while the display is on,
-    then power off after 30 s
-  - `FlipDot Display Power`: off = clear the display and power it off after 1 s
+- **Home Assistant** through MQTT (`home-assistant-integration` library). The switch and
+  the dropdown report their new state back, so Home Assistant always shows the real state.
+  - `FlipDot Display Power` switch: off = clear the display and power it off after 1 s
+  - `FlipDot Display Mode` dropdown: the page shown during the day, `Clock` or a bitmap.
+    The options follow the bitmaps saved in the web UI (`net/ha_page_select.*`, because
+    ArduinoHA's `HASelect` can't change its options).
+  - `FlipDot Display Off image` dropdown: `Blank` or a bitmap, drawn instead of a blank
+    screen at night and when the Power switch turns the display off
+  - `FlipDot Display IP` sensor: the IP address
 
-  Plus the **FlipDot Display Firmware** sensor (see
+  The old `FlipDot Display Clock` switch is removed from Home Assistant on connect (empty
+  retained discovery config). Plus the **FlipDot Display Firmware** sensor (see
   [Keeping OTA alive](#keeping-ota-alive)) and the **FlipDot Display Recovery** button,
   which restarts into the recovery app for 10 minutes.
 - **Relay on GPIO 13** switches the power for both the panels and the Nano. `LOW` turns it