Add Home Assistant controls over MQTT

Upgrade ArduinoHA from 1.3.0 to 2.1.0 and rewrite mqtt.ino for it.
The clock now shows up in Home Assistant (discovery prefix
"homeassistant") with:
- switches Tubes (blank the display, runtime only) and Auto dim
- numbers Day/Night brightness
- light Backlight (on/off, brightness, RGB mapped to the colour wheel)
- select LED effect, button Cathode protect
- sensors Light (LDR) and WiFi signal, plus the upstream sensors when
  the clock has them; master-clock subscriptions are kept

Settings changed from HA are saved to EEPROM 3 s after the last change.
Fixes the unbounded memcpy in the MQTT message handler.

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

firmware/uniclock/README.md

@@ -7,6 +7,8 @@ Peter Gautier (GPL-3.0-or-later), vendored from upstream commit `1189810` (2026-
   It keeps the upstream layout, so it still opens in the Arduino IDE.
 - `lib/`: the library versions bundled upstream (`libraries/*.zip`), only those the ESP8266 build
   needs. The upstream manuals, datasheets, photos and ESP32-only libraries were left out.
+  Exception: `arduino-home-assistant` is v2.1.0 (upstream bundles 1.3.0, which has no light,
+  number, select or button entities); `mqtt.ino` was rewritten for it.
 - `platformio.ini`: PlatformIO build for the ESP-12 (4 MB flash, 2 MB SPIFFS).
 
 Local changes are separate commits on top of the import (`git log -- firmware/uniclock`).