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>

tests/test_sse_subscription.py

@@ -351,24 +351,6 @@ async def test_stream_events_yields_documented_frames_and_sends_bearer() -> None
     assert call["json"] is None
 
 
-async def test_stream_events_reports_household_timezone() -> None:
-    """The SSE GET carries Home Assistant's zone beside the Bearer token
-    (task 6351b36d), like the snapshot and completion requests."""
-    frame = _frame_text(*DOCUMENTED_FRAMES[0])
-    transport = _StreamTransport(_StreamingResponse(200, [frame]))
-    client = NestQuestApiClient(
-        BASE_URL, TOKEN, transport, household_timezone="America/New_York"
-    )
-
-    frames = [frame async for frame in client.stream_events()]
-
-    assert frames == [DOCUMENTED_FRAMES[0]]
-    assert transport.calls[0]["headers"] == {
-        "Authorization": f"Bearer {TOKEN}",
-        "X-NestQuest-Timezone": "America/New_York",
-    }
-
-
 async def test_stream_events_buffers_lines_across_chunks_and_joins_data() -> None:
     """Chunk-split lines parse, and multi-line data joins per SSE rules."""
     first_type, first_payload = DOCUMENTED_FRAMES[0]