Dragging a keyframe moved it a frame or two and then let go: the key's widget id includes its
time, so after the first move it was a new widget and egui ended the drag. The Timeline now
follows the whole gesture itself (`KeyDrag`: from where the button went down, by whole frames,
every selected key, one undo step), like After Effects. Holding Shift snaps the grabbed key to
the current time, the work area and composition markers.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
