Promote to production: onet connecting line fix - #307

The line was drawn but invisible: strokeWidth 0.14 combined with
vectorEffect='non-scaling-stroke' meant 0.14 SCREEN pixels, not 0.14
cells. The overlay was also mispositioned — its hand-picked -8%/116%
offsets didn't match the viewBox (8 columns needs -12.5%/125%) and
ignored the grid's gap and padding.

Now the path is converted to pixel centres measured from the real cells
(extrapolating the cell pitch for the one-cell margin), drawn in the
grid's own pixel space with overflow visible, so it lines up exactly at
any board size and routes that leave the board still show. Added a dark
halo behind the line plus endpoint dots for contrast on any tile, and
lengthened the flash from 240ms to 420ms so it is actually seen.