Fix bbox cache invalidation when moving objects between groups in Objects dialog

Manual Test Case

Purpose:
Verify that moving an object between groups via Objects dialog menu correctly invalidates the source group’s cached bounding box and triggers a display update.

Steps

• Create two groups with 2 shapes each. Lets call them Group A (source) and Group B (destination).
• Drag a shape from Group A and drop it into Group B in the Objects dialog.
• Observe the canvas and selection.

Result with patch

• Group A’s bounding box updates immediately after the shape is removed.
• No incorrect bounding box remains.
• No additional interaction is required to refresh the display.

Result without patch

• Group A keeps its old bounding box until another action forces a redraw.

Fixes #5996 (closed)