An intuitive way to add past habits

Build in public day 18/30: Still working through the React rewrite, but also thinking through how to make it easier to add past habits. This would be useful e.g. to fix a streak if it gets missed since the X API is still spotty, or maybe you just for got to log it that day.

Here you'd just click a day on the habit graph, the available habits would pop up and you could check it off. Simple and intuitive.