Dashboard: Fill in calendar automatically, improve display

I want the calendar to automatically fill in a reading day if I have recorded a reading session / reading activity on one of my books.
(In the far future, maybe even displaying pages read on the calendar, or maybe color-code the calendar like an Anki heatmap? And also pages read over time stats, not just days read or hours recorded)

See Frontend Development Roadmap - #32 by Myria

The automatic filling is now live, so I will close this card and move the “far future” suggestions to a new card.