feat(plan): device markers are landmarks in the Plan editor

The Plan editor hid every device marker (display: none), so walls,
openings and stairs were placed against devices from memory. It now
shows them exactly as the Background editor does (#362): the same
markers as View, at the Background editor's effective opacity, fully
pointer-inert. The fade is filter: opacity(0.35) on each marker, not
on .devlayer, because room labels and room settings buttons share that
layer and stay opaque and interactive; filter multiplies the marker's
own opacity (.unavail -> 0.35 x 0.35), as the Background layer does.
Marker handlers already fail closed outside View/Devices.

demo/smoke_plan_device_landmarks.mjs proves AC1/AC2: same markers as
View, per-marker alpha equal to Background, core and capsule fall
through, a real click on a marker reaches the Walls tool and a room
settings button below, cursor and target unchanged, handlers inert, no
tabindex. Three mutants guard hiding, opacity override and the pointer
boundary. UX-MODES, DECOR-EDITOR, USER-GUIDE.ru mode table, changelogs.

Issue: #687
User-Visible: yes
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

docs/UX-MODES.md

@@ -172,6 +172,11 @@ tabs, the editor-close reserve, zoom and one gear whose menu holds the rest.
   editor with the draw tool armed (an empty floor has nothing useful in View).
 - ⚙ General settings (fill palette) lives here — it is about the plan's appearance.
 
+- Devices are landmarks here exactly as in Background (#687, #362): the same
+  markers as View at 35% (each marker's own opacity multiplied, as the
+  Background layer does) and fully pointer-inert — every press, hover and
+  cursor belongs to the Plan tool, room label or room settings button below.
+  Room labels share the device layer and stay opaque and interactive.
 - Independent partitions and columns are masonry for hit testing as well as
   area/light: room hover stops at their physical bodies just as it stops at a
   thick room wall. This does not split the room or change its HA area.