Remove every automatic timezone-setting mechanism (task 7f685179)

The application timezone is now only ever set by an admin saving it:

- API: drop adopt_reported_timezone; the panel household clock reads the
  stored settings only and ignores any timezone request header.
- Integration: stop reporting Home Assistant's time_zone in an
  X-NestQuest-Timezone header (snapshot, completion and SSE requests).
- Admin app: remove the browser-zone detection and the automatic PATCH
  on load; the timezone field stays a manual control.
- Remove the timezone_configured marker from the core settings, the
  settings store and the admin settings API. A stored document that
  still carries the key loads harmlessly (unknown keys are ignored).

Tests replace the adoption expectations with proofs that a panel
timezone header never changes the stored settings, a fresh install's
empty zone stays empty with no settings write, the admin settings carry
no timezone_configured, and a manual admin timezone PATCH persists.

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

custom_components/nestquest/core/const.py

@@ -114,24 +114,11 @@ CONF_CELEBRATION_ENABLED = "celebration_enabled"
 DEFAULT_AUTOMATION_ENABLED = True
 # The household's IANA time zone (e.g. ``America/New_York``) the API
 # service reads its clock in.  The empty string is the API host's own
-# local time.  While the admin has not chosen a zone
-# (``timezone_configured`` is ``False``) the API adopts the zone the
-# integration reports from Home Assistant's configured ``time_zone``
-# (:data:`PANEL_TIMEZONE_HEADER`), so the default is the household's
-# zone without any manual setup.  The integration itself always uses
+# local time.  Only an admin sets it (manually, from the admin app);
+# nothing sets it automatically.  The integration itself always uses
 # Home Assistant's ``time_zone`` and ignores this field.
 CONF_TIMEZONE = "timezone"
 DEFAULT_TIMEZONE = ""
-# Whether the admin has made an explicit ``timezone`` choice.  ``False``
-# (a fresh install) lets the API follow Home Assistant's reported zone
-# and the admin UI auto-set the browser's zone once while none is
-# stored; ``True`` pins the stored value — including an explicitly-chosen
-# empty ``timezone`` (the API host's local time) — so neither changes it.
-CONF_TIMEZONE_CONFIGURED = "timezone_configured"
-DEFAULT_TIMEZONE_CONFIGURED = False
-# The panel-plane request header carrying Home Assistant's configured
-# IANA ``time_zone`` from the integration to the API service.
-PANEL_TIMEZONE_HEADER = "X-NestQuest-Timezone"
 
 # Panel-plane API client (Feature 18): the integration reaches the
 # NestQuest API service (Feature 16) over HTTP instead of the database.