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/lib/arduino-home-assistant/library.properties

@@ -0,0 +1,10 @@
+name=home-assistant-integration
+version=2.1.0
+author=Dawid Chyrzynski <dev@chyrzynski.pl>
+maintainer=Dawid Chyrzynski <dev@chyrzynski.pl>
+sentence=Home Assistant MQTT integration for Arduino
+paragraph=Lightweight library that provides easy to use API for integrating your Arduino/ESP based device with Home Assistant.
+category=Communication
+url=https://github.com/dawidchyrzynski/arduino-home-assistant
+architectures=*
+depends=PubSubClient