ClickableSlider previously accepted the mouse press without forwarding
to QSlider, so the drag mechanism never started. Now setSliderPosition
is called first (jumping the handle to the click point) and the event
is always forwarded so Qt enters normal drag mode. mouseMoveEvent is
also overridden to keep snapping to the cursor during a drag.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>