Update timer used fixed ms-until-midnight computed at import. Once
midnight passed, the fixed interval re-armed an elapsed timeout, busy
looping update_calendar and destroying/recreating every label until
GTK3 stopped rendering them. Timer now recomputes the interval each
day and the grid mutates pre-built labels in place via set_label /
set_name instead of rebuild.
Also added missing year check for today highlight.
sankalp