Implement disconnection feedback UI and drag logic

[kh-ub-ayb]

Build the UI overlay that instantly renders a grey footprint at the exact detachment socket when a brick is ripped out of a tower, providing context of where the brick came from.

This requires capturing the exact detachment socket prior to the tower split, and handling edge cases where a nested parent brick cavity collapses.

Objectives:

• Create DisconnectShadowView.tsx to render a ghost footprint at the detachment source
• Add unit tests for DisconnectShadowView
• Update Workspace.tsx to render the disconnect shadow
• Wire up detachment logic in useBrickMove to populate the disconnect shadow in the store
• Secure the dragStateRef lock during drag start to prevent React re-renders from prematurely clearing the shadow