feat: interactive section planes with TransformControls gizmo

Replace static PlaneHelper wireframes with transparent colored meshes
(X=red, Y=green, Z=blue) and a shared TransformControls gizmo for
real-time drag-to-clip interaction. Camera controls lock during drag,
clicking switches between planes, clicking outside deselects.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jochem25