fix(org-chart): separate fixed and expanded canvas backgrounds

packages/org-chart/src/styles/org-chart-content.css

@@ -3,8 +3,9 @@
    The stage keeps its natural width (tree + side) and is uniformly scaled by Core JS;
    there is no reflow breakpoint and no inner horizontal scroll. Column/group gaps are
    screen-only layout values (no data or parentId change). */
-.hm-orgchart-composition { position: relative; width: 100%; min-width: 0; overflow: hidden; }
+.hm-orgchart-composition { position: relative; width: 100%; min-width: 0; overflow: hidden; background: var(--hm-orgchart-fixed-canvas-background, transparent); }
 .hm-orgchart-stage {
+  background: var(--hm-orgchart-fixed-canvas-background, transparent);
   /* Engine padding (32px) + node margins (16px) already separate the six 252px
      organization columns; the tree wrapper adds no extra width. */
   --hm-orgchart-column-gap: 0px;