Graphical seat map: clicking a seat no longer books it (pointer capture swallows the click)

Follow-up to #3611792 (graphical seat-map picker).

The map pan handling calls viewport.setPointerCapture() on pointerdown so a drag started on the map keeps receiving pointer events when the cursor leaves the viewport. But capturing the pointer on pointerdown prevents the following click event from reaching the seat button in Chrome, so clicking a seat does nothing and no booking happens.

Fix: drop setPointerCapture() and instead listen for pointerup and pointercancel on the window, so a release outside the viewport still ends the drag while the click still reaches the seat. The existing "no button pressed" guard on pointermove is kept as a further safety net.