fix: synchronize orthographic zoom across VS Code viewers #53

Follow-up to PR #50 and the review finding.

MatterViz defaults to an orthographic camera and emits camera_zoom on wheel zoom. The viewer's shared camera state currently retains only position and target, so panes with "sync cameras" enabled do not share the same zoom scale.

Carry the zoom value in CameraPose, capture it in on_camera_move, and apply it to each viewer's scene props. Verify reset and toggling sync as well.

Acceptance: wheel-zooming one pane updates the scale of the other synchronized panes; unsynchronized panes remain independent.

Explicitly deferred from PR #50 at the user's request. Rotation/panning sync remains available; zoom must currently be adjusted per pane.