A scratch test earlier in development wrote a fake 42s Beginner record into
the legacy ~/.config/minesweeper/leaderboard.ini, which the migration imported;
as a stored best it can never be beaten by slower real games, so the popover
showed a lower time than actually played.
- Add a 'Clear scores' button to the leaderboard popover
- Delete the legacy file after a successful migration so stale data can't
be re-imported