add support for Vox (Amuse Labs compatible) download by date via calendar picker

[superhawk610]

I spent some time poking around Vox's crossword archive and found that it references /calendar-picker, in a similar manner to the existing /date-picker resource used by other Amuse Labs puzzles. This resource doesn't provide JSON-formatted info like the other, but it does allow selecting a year/month via URL parameters, and provides a day->puzzle ID mapping after a bit of HTML parsing.

I've used this to implement find_by_date for AmuseLabsDownloader, and provided the required calendar_url for the Vox downloader. I haven't tested with other Amuse Labs puzzles, but I imagine that calendar_url shouldn't be too hard to find to enable this for others (maybe we should just use picker_url and replace date-picker with calendar-picker?

To test that this works:

xword-dl vox --date 2025-12-31

and that missing puzzles are also handled gracefully:

# others exist in the same month, but none for this day
xword-dl vox --date 2024-12-29

# none exist in this month at all
xword-dl vox --date 2002-01-01