Add Baby Tracker client and Home Assistant integration

Python client for the Baby Tracker sync server, reverse engineered from
the Android app (4.53), and a HACS-installable Home Assistant integration
built on it. Each baby gets a log calendar and summary sensors, and actions
add entries that sync back to the group.

Reading has been run against the real server. Uploading and running inside
real Home Assistant have not.

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

tests/ha_stubs/homeassistant/util/dt.py

@@ -0,0 +1,21 @@
+from datetime import datetime, timezone
+
+DEFAULT_TIME_ZONE = timezone.utc
+
+
+def utcnow():
+    return datetime.now(timezone.utc)
+
+
+def as_utc(value):
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=DEFAULT_TIME_ZONE)
+    return value.astimezone(timezone.utc)
+
+
+def as_local(value):
+    return value.astimezone(DEFAULT_TIME_ZONE)
+
+
+def start_of_local_day():
+    return utcnow().astimezone(DEFAULT_TIME_ZONE).replace(hour=0, minute=0, second=0, microsecond=0)