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/binary_sensor.py

@@ -9,7 +9,7 @@ snapshot (the same ChildDaySnapshot the day sensors read):
   celebration automation must never fire on an empty day (the
   day-complete event carries the same rule, Feature 10 task 7).
 - ``binary_sensor.nestquest_<child>_present_today`` — ON when the
-  presence engine resolves the child present on the HA-local date
+  presence engine resolves the child present on the household-local date
   (schedules and overrides, D-004 anchor-date arithmetic).
 
 Naming and registry identity follow the day sensors' contract: names