fix(onet): make the connecting line visible

[slaveofcode]

You were right — no line appeared on a match. Two bugs:

1. The line was invisible, not missing. strokeWidth={0.14} combined with vectorEffect="non-scaling-stroke" means 0.14 screen pixels, not 0.14 cells. It was rendering the whole time, just sub-pixel thin.

2. The overlay was mispositioned. Its hand-picked -8% / 116% offsets didn't match the viewBox (8 columns needs -12.5% / 125%) and ignored the grid's 2px gap and padding — so even at a visible width it would have been off.

Fix: the path is now converted to pixel centres measured from the real cells (extrapolating the cell pitch to handle the one-cell margin) and drawn in the grid's own pixel space with overflow-visible. That lines up exactly at any board size or difficulty, and routes that leave the board still render. Added a dark halo behind the line plus endpoint dots so it reads on any tile colour, and lengthened the flash 240ms → 420ms.

Verified in a browser, not just asserted: captured mid-flash, the polyline renders 101.5,34.5 → 101.5,-32.5 → 369.5,-32.5 → 369.5,34.5 — a 2-turn route going up out of the board and back down (negative y = the outside margin), 268×67px.

Note: because a route can legitimately travel outside the board, the flash briefly overlaps the stat chips above the grid. I left it — it actually communicates the outside-border rule, and reserving a full cell of clearance would cost ~67px of permanent dead space. Happy to add clearance if you'd rather.

• Tests: 1731 passing · Lint: 0 errors · Build green