Use bold text for month names in the month view's 1st-day cell

At least use bold type to make them easier to spot while scrolling, though it doesn't go as far as using a bigger font size (as it might cause misalignment) or color.

This pretty much matches the minimum acceptable approach for @snwh and @PhilippSauberz in #962 (comment 1819406) (I saw no visual difference between weight 700 and weight bold, so in that case I prefer to pick the explicitly named one).

Before vs after:

It's not as much as I'd like, but it's a start / mergeable MVP, I think.

---

Future ideas

For one thing I'd love to have full month names (instead of abbreviations) when there is enough space, and I even made a proof-of-concept commit for that in this branch, shown here with month name labels shown on every cell just for testing purposes / to ensure the 5-days range trick actually matched the weather forecast area (that testing trick is not in the committed code):

...but I suspect that particular implementation would be considered a naughty hack (it 100% works in practice, though) so I'm not making a MR out of that one unless someone tells me it would be worthwhile; besides, surely @camelCaseNick could come up with a much smarter solution as part of !309 (and ideally then show the full month names when possible)...