Import ESP_UniClock (upstream 1189810)

Vendored from https://github.com/gpeter62/ESP_UniClock: the sketch
source, the web UI and the libraries the ESP8266 build needs, unpacked
from the bundled zips. Adds a PlatformIO project for the ESP-12. The
source is unchanged; local changes follow as separate commits.

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

firmware/uniclock/lib/arduino-home-assistant-main/examples/availability/README.md

@@ -0,0 +1,33 @@
+# Home Assistant availability example
+
+This example shows how to use "availability" feature of the Home Assistant.
+It's supported only by following device types:
+* Binary sensor
+* Sensor
+* Switch
+* HVACs
+* Fans
+
+## Initialization and usage
+
+Availability feature is turned off by default. In order to turn it on for the specific
+sensor/switch you need to set initial state of the unit by calling `setAvailability` method.
+
+Example:
+```cpp
+...
+HABinarySensor sensor("input", "door", true, mqtt);
+
+void setup() {
+    // ...
+
+    // method must be called before `mqtt.begin(...)`
+    sensor.setAvailability(false); // offline
+
+    mqtt.begin(BROKER_ADDR);
+}
+...
+```
+
+After availability is set for the very first time you can call `setAvailability` in any time.
+Each time the method is called, the device will publish MQTT message to the broker.