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/snapshot.py

@@ -18,7 +18,7 @@ the panel payload shaper (:func:`instance_payload`), and the async
 service's panel snapshot route share ONE implementation of both the
 snapshot assembly and the entity-facing attribute payload shape.
 Nothing here imports :mod:`homeassistant`; the builder takes the
-database, the settings object, and the HA-local ``now`` (a single
+database, the settings object, and the household-local ``now`` (a single
 clock read the caller already resolved, from which it derives
 ``today``) and returns the snapshot, mirroring exactly what the
 coordinator used to do inline.
@@ -187,7 +187,7 @@ async def build_snapshot(
     :func:`instance_payload` layer — this builder produces the shared
     shape both consumers read.
 
-    ``now`` is the HA-local timezone-aware datetime the caller already
+    ``now`` is the household-local timezone-aware datetime the caller already
     resolved (a SINGLE clock read); the overdue check compares
     ``now.time()`` against the instance's ``due_time`` for today's open
     instances.  The builder performs NO internal clock read — it never