Header now distinguishes click vs drag:
- mousedown → mouseup with <4px total movement = treated as a CLICK,
toggles collapse (same as clicking the arrow)
- More movement = actual drag, keeps existing behavior
- Threshold avoids accidental toggling during tiny mouse jitters
Also enlarged the collapse arrow button: bigger font, padded box with
border + background tint, easier to hit. Either gesture (header click
or arrow click) does the same thing now.