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/styles.ts

@@ -432,6 +432,48 @@ export const cardStyles = css`
       user-select: none;
       z-index: 3;
     }
+    /* decor (background) layer */
+    .decorlayer .dshape { pointer-events: none; }
+    .stage.mode-decor .decorlayer .dshape {
+      pointer-events: visiblePainted;
+      cursor: pointer;
+    }
+    .stage.mode-decor.dtool-select .decorlayer .dshape { cursor: move; }
+    .decorlayer .dsel {
+      filter: drop-shadow(0 0 3px var(--hp-accent));
+    }
+    .decorlayer .ddraft {
+      opacity: 0.75;
+      stroke-dasharray: 6 5;
+      pointer-events: none;
+    }
+    .decorlayer text {
+      font-weight: 600;
+      user-select: none;
+      dominant-baseline: middle;
+      text-anchor: middle;
+    }
+    .stage.mode-decor {
+      outline: 2px solid #26a69a;
+      outline-offset: -2px;
+    }
+    .stage.mode-decor.dtool-line, .stage.mode-decor.dtool-rect,
+    .stage.mode-decor.dtool-ellipse, .stage.mode-decor.dtool-text {
+      cursor: crosshair;
+    }
+    .stage.mode-decor .room, .stage.mode-decor .devlayer { pointer-events: none; }
+    .stage.mode-decor .oplock { pointer-events: none; }
+    .decorbar .dcolor {
+      width: 30px; height: 26px; padding: 0; border: none; background: none; cursor: pointer;
+    }
+    .decorbar .dwidth {
+      font-family: inherit; font-size: 12px; border-radius: 6px;
+      background: var(--hp-bg2, transparent); color: var(--hp-txt); border: 1px solid var(--hp-muted);
+      padding: 3px 5px;
+    }
+    .decorbar .dfill {
+      display: inline-flex; align-items: center; gap: 4px; font-size: 12px; cursor: pointer;
+    }
     .opghost {
       stroke: var(--hp-open, #ff9800);
       stroke-width: 5;