feat v1.33.0: background editor — visual decor layer

- third mode tab: draw lines/rects/ovals/text with grid snap, drag
  preview; select+move (Delete key), erase, color/width/fill controls;
  Esc ladder; text dialog with S/M/L sizes and dblclick re-edit
- decor renders under rooms, visible in every mode, click-transparent
  outside its editor; stored in space.decor with backend schema
  (line/rect/ellipse/text variants), rev/optimistic locking as usual
- smoke_decor.mjs (19 checks); smoke_editor_tabs updated to 3 tabs;
  TESTING/CHANGELOG/UX-MODES same-commit

docs/UX-MODES.md

@@ -12,7 +12,10 @@ A segmented control in the card header with three tabs; the active one is visual
 highlighted, and edit modes add a colored frame around the stage so the mode is
 obvious at a glance:
 
-**[ 📐 Plan editor ] [ 🔧 Device editor ]** — View has NO tab (since v1.30.2).
+**[ 📐 Plan editor ] [ 🔧 Device editor ] [ ✏️ Background editor ]** — View has
+NO tab (since v1.30.2). The Background editor (v1.33.0) manages a purely visual
+decor layer (lines/rects/ovals/text in `space.decor`, drawn under the rooms,
+inert everywhere outside its editor).
 
 - **View** is the implicit default state: no editor tab is active. It is the only
   state after every load (edit modes are never restored across reloads).