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/conftest.py

@@ -1017,8 +1017,8 @@ def make_hass() -> tuple:
     hass.bus = EventBus()
     hass.http = HttpRegistry()
     hass.extra_js_urls: list[str] = []
-    # A valid IANA time zone (the day-rollover listener reads it to
-    # compute "today" in HA local time).
+    # A valid IANA time zone for the fake Home Assistant config (the
+    # NestQuest application timezone is the admin-set setting, not this).
     hass.config.time_zone = "UTC"
 
     # The minimal entity registry: entities self-register through