month-cell: For 1st day of the month, use bold style for month name & day number labels

Make it easier to visually track which exact month boundary you are crossing while scrolling, by making the text bold:

Light mode	Dark mode

Minimally invasive change that can meet @PhilippSauberz & @snwh's MVP wishes in #962 (they requested that not only the month name is bold, but also the day number).

Experimental follow-up: !479