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>

tests/test_completion.py

@@ -834,7 +834,7 @@ def test_complete_instance_no_due_time_after_due_date_is_late(
 
 
 def test_complete_instance_ha_local_wall_clock_not_utc(tmp_path) -> None:
-    """HA-local 11:00 with due_time 12:00 is on time even when UTC is 19:00.
+    """Household-local 11:00, due 12:00, is on time even when UTC is 19:00.
 
     The helper's zone is UTC-8, so 11:00 local is 19:00 UTC.  Converting
     to UTC before comparing would mark this late.