Enable serif/sans-serif font switching with fallback for unsupported languages

[chroju]

I’ve noticed that the current font options do not support Japanese characters. While using diverse fonts for Japanese may not be necessary, I would like the option to switch between serif and sans-serif fonts. Ensuring that the font-family settings always include "serif" or "sans-serif" would help display text appropriately even for languages not supported by the specified fonts.

Thank you for developing this amazing application!