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

src/i18n/en.json

@@ -261,5 +261,24 @@
   "roomcard.light_on": "On",
   "roomcard.light_off": "Off",
   "roomcard.light_partial": "{on} of {total}",
-  "toast.split_pick_inside": "Intermediate cut points must be inside the room"
+  "toast.split_pick_inside": "Intermediate cut points must be inside the room",
+  "mode.decor": "Background editor",
+  "decor.select": "Select",
+  "decor.line": "Line",
+  "decor.rect": "Rectangle",
+  "decor.ellipse": "Oval",
+  "decor.text": "Text",
+  "decor.erase": "Erase",
+  "decor.color": "Color",
+  "decor.width": "Line width",
+  "decor.w_thin": "Thin",
+  "decor.w_mid": "Medium",
+  "decor.w_thick": "Thick",
+  "decor.fill": "Fill",
+  "decor.text_title": "Text label",
+  "decor.text_label": "Text",
+  "decor.text_size": "Size",
+  "decor.size_s": "Small",
+  "decor.size_m": "Medium",
+  "decor.size_l": "Large"
 }