todayISO() and day.js's stepDay() formatted dates via toISOString(), which
is UTC. For UTC+ viewers past local midnight this rolled the date a day
early: "today", the date-input max, the next-day disabled guard, and the
Weekly "Today" button all referred to the wrong day, and prev/next
navigation stepped off-by-one.
Route both through the existing local isoOfDate() so every view (Weekly,
Day Detail, Calendar) agrees on the viewer's local day.