room settings button: room-centred, icon-sized, zooms with the plan

Owner's spec: the button is no longer glued to the room NAME (which the user
can drag anywhere) — it anchors to the geometric centre of the ROOM
(interiorPoint for polygons, so an L-shaped room gets a point actually
inside it), one button-height below centre so it never covers the name,
whose default position is that same centre. Height is 70% of a device icon
box, and since --icon-size already rescales with the view, the button zooms
with the plan instead of keeping a constant screen size (verified: x2.2 zoom
-> x2.20 button). The small metric rows under the room name (temperature,
humidity, signal, lights) now render in the plan editor too — they used to
be view-mode only.

smoke_room_cards updated: plainInPlan now asserts metrics ARE present in the
editor (the old assertion pinned the old behaviour), plus gearDetached.

docs/TESTING.md

@@ -239,6 +239,11 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       on the plan after a reload. Same for each tap action and each fill mode
       [auto: backend test_every_display_mode_the_editor_offers_is_accepted and
       neighbours, test_a_marker_showing_its_value_can_be_saved]
+- [ ] Room settings button (dev): detached from the (movable) name label —
+      always at the room's geometric centre, one button-height below it; sized
+      at 70% of a device icon and zooming WITH the plan; the small metric rows
+      under the room name now show in the plan editor too
+      [auto: smoke_room_cards gearDetached/plainInPlan]
 - [ ] Hide-from-plan flag (dev, docs/FILTERING.md): every device dialog has
       the checkbox, incl. virtual; hidden devices vanish from every mode and
       the count, still count toward room LQI, cast no glow/light fill; the