The timeline's terminating dot used a single left offset (-17px) at both
breakpoints, but the thread's padding-left differs (22px desktop, 18px
mobile) and the step dots already carry a per-breakpoint offset. The 6px
dot therefore landed 2px right of the 2px rail on desktop and 2px left of
it on mobile, which is the visible kink under an expanded last step.
Derive each offset from the rail's centre instead: the rail sits at
left:5px and is 2px wide, so the dot's left edge belongs at 3px, giving
3px - padding-left per breakpoint.
