Document the manual, admin-set application timezone (task e9aa78f3)

Add an "Application timezone" section to docs/ARCHITECTURE.md: the
timezone is set only by an admin from the admin app (GET/PATCH
/api/v1/admin/settings), is the single source of local time for the
panel, completions, the missed-sweep scheduler and the admin app, means
the API host's local zone while empty (published as the effective zone),
and an admin save of timezone or day_rollover_time re-plans the sweep.

Correct docstrings and comments that still described the local date as
Home Assistant's ("HA-local", read from hass.config.time_zone by the
integration) or the stored zone as reported from Home Assistant.
Comment and docstring changes only; no behaviour change.

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

custom_components/nestquest/core/settings.py

@@ -316,12 +316,11 @@ class NestQuestSettings:
 
         The API service does not run inside Home Assistant, so its one
         clock read (an aware host-local ``now``) is converted here: with
-        :attr:`timezone` set — the admin's choice, or else the zone the
-        integration reports from Home Assistant — the SAME instant is
-        re-expressed in that zone, so ``.date()`` and ``.time()`` are the
-        household's wall clock even when the host runs UTC.  With
-        :attr:`timezone` empty the instant is returned unchanged (the
-        host's local time).
+        :attr:`timezone` set — only ever by an admin, from the admin
+        app — the SAME instant is re-expressed in that zone, so
+        ``.date()`` and ``.time()`` are the household's wall clock even
+        when the host runs UTC.  With :attr:`timezone` empty the instant
+        is returned unchanged (the host's local time).
         """
         if not self.timezone:
             return instant
@@ -341,7 +340,7 @@ class NestQuestSettings:
         """Return the rolling horizon window ``[today, today + horizon_days]``.
 
         ``today`` is REQUIRED (there is no host-clock default): the
-        integration passes the HA-local ``hass.config.time_zone`` date so
+        API passes the household-local date (:meth:`household_now`) so
         the horizon tracks the household's own day, and a silent
         host-clock fallback would be a wrong-day footgun (a test or a
         caller that forgets to pass ``today`` would otherwise materialize