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>

api/routes_admin.py

@@ -165,13 +165,12 @@ integration's Home-Assistant config-entry options remain a temporary,
 SEPARATE source until Feature 18 wires the client to the API — the two
 are deliberately not unified here:
 
-- ``GET /settings`` returns the current effective settings: all twelve
+- ``GET /settings`` returns the current effective settings: all eleven
   documented fields (the rolling horizon, the day rollover time, the
   notify target, the three notification times, the four enable
-  toggles, the ``timezone`` — the IANA household zone the
-  household-local dates derive from, empty meaning host local — and
-  ``timezone_configured``, whether the admin explicitly chose that
-  zone) — the defaults on a fresh database.
+  toggles, and the ``timezone`` — the IANA household zone the
+  household-local dates derive from, empty meaning host local; only an
+  admin's ``PATCH`` ever sets it) — the defaults on a fresh database.
 - ``PATCH /settings`` updates ONLY the supplied fields: the body is a
   JSON object of settings field names to new values, passed STRAIGHT
   to :func:`nestquest_core.settings_store.update_settings` — the
@@ -906,7 +905,7 @@ class AdminSettingsResponse(BaseModel):
     """The effective settings as the admin plane serializes them.
 
     Mirrors the core :class:`~nestquest_core.settings.NestQuestSettings`
-    — all TWELVE documented fields, resolved (every absent stored field
+    — all ELEVEN documented fields, resolved (every absent stored field
     already fell back to its default in the core).  The same shape both
     the GET and the PATCH route answer with.
     """
@@ -924,9 +923,6 @@ class AdminSettingsResponse(BaseModel):
     #: The household IANA time zone the API reads its clock in; ``""``
     #: means the API host's local time.
     timezone: str
-    #: Whether the admin explicitly chose ``timezone`` (even ``""``);
-    #: ``False`` lets the admin UI auto-set the browser's zone once.
-    timezone_configured: bool
 
 
 class AdminMissedSweepResponse(BaseModel):
@@ -2183,7 +2179,7 @@ def _settings_response(
     """Serialize one core NestQuestSettings into the documented payload.
 
     Explicit field-by-field (not ``dataclasses.asdict``) so the payload
-    stays pinned to the twelve documented fields even if the core
+    stays pinned to the eleven documented fields even if the core
     dataclass later grows another one.
     """
     return AdminSettingsResponse(
@@ -2198,7 +2194,6 @@ def _settings_response(
         end_of_day_report_enabled=settings.end_of_day_report_enabled,
         celebration_enabled=settings.celebration_enabled,
         timezone=settings.timezone,
-        timezone_configured=settings.timezone_configured,
     )
 
 
@@ -2296,7 +2291,7 @@ async def admin_history_csv(
     response_model=AdminSettingsResponse,
 )
 async def admin_get_settings(request: Request) -> AdminSettingsResponse:
-    """Return the current effective settings (all twelve documented fields).
+    """Return the current effective settings (all eleven documented fields).
 
     Requires a valid admin JWT (the router's shared
     :func:`~api.auth.require_admin` dependency — the ONE check; this