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/TESTING.md

@@ -140,6 +140,13 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       (explicit ripple color still wins); off/white lights unchanged [auto]
 - [ ] Alarm pulse (v1.27.0): leak/smoke/gas/CO/siren in 'on' pulse a red ring over any
       display mode; clears on 'off'; unavailable never alarms [auto]; reduced-motion static
+- [ ] Background editor (v1.33.0): third tab with its own toolbar (select /
+      line / rect / oval / text / erase + color, width, fill, X); shapes drag-
+      drawn with grid snap and live preview; degenerate shapes dropped; text
+      via dialog (S/M/L, color; dblclick re-edits); Select moves (snap), Delete
+      removes, Erase deletes on click; Esc: draft → selection → select tool →
+      View; decor renders under rooms, visible everywhere, inert outside the
+      editor; stored in space.decor (rev/lock, backend schema) [auto]
 - [ ] Opening hover preview (v1.32.1): with the Opening tool, hovering near a
       wall shows a dashed 90 cm ghost snapped onto the wall (with a center
       dot); no ghost far from walls, over an existing opening (click = edit),