Why the goal is 90 days
A short goal ends before a habit forms. A long one feels out of reach. Ninety days sits in between.
The bar under your streak fills toward 90 days. It gives the streak a finish line you can see from the first day.
Ninety days is long enough for a task to stop feeling new, and short enough that the end is in sight within a season.
Link Line on the wallpaper draws the same 90 days as a grid of dots, one for each day, with a line running through the days so far and a cross on any day the streak broke.