Adopt Home Assistant's time zone for the API household clock

The deployed API host runs UTC and the household `timezone` setting was
never set, so the panel snapshot read UTC: morning quests showed overdue
hours early and `today_iso` rolled over at UTC midnight.

The integration now reports `hass.config.time_zone` on every panel
request (`X-NestQuest-Timezone`).  While the admin has not chosen a zone
(`timezone_configured` is false) the API adopts and persists it, so the
panel, the admin plane, and the missed-sweep scheduler all read the
household clock out of the box.  An explicit admin choice — including an
explicitly chosen empty zone — still wins; an invalid reported zone is
logged and ignored.  No schema change.

Task: 6351b36d-0eef-4e7d-bf5a-96b754cbc883

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

custom_components/nestquest/coordinator.py

@@ -372,7 +372,10 @@ def coordinator_client_from_entry(
         return _UnconfiguredApiClient()
     if session_factory is not None:
         return NestQuestApiClient(
-            base_url, panel_token, session_factory(hass)
+            base_url,
+            panel_token,
+            session_factory(hass),
+            household_timezone=hass.config.time_zone,
         )
     from custom_components.nestquest.api_client import client_from_entry