feat: use system UI font, fall back to sans-serif
feat: use system UI font, fall back to sans-serif

Historically Patchwork has used a default sans-serif font and used a
macOS-specific font for some inputs. This was removed in a recent
release but @ahdinosaur pointed out that it caused serif fonts on his
machine. Bad! 😱

This commit switches us back to a default font, with a few changes:

- we use the UI font of the OS if possible
- we fall back to a sans-serif if we can't detect one
- <textarea> elements are now monospaced again, the browser default
  - this is subjective but I think it makes writing Markdown easier
  - I'm inclined to use browser defaults unless there's a reason not to

ssbc/patchwork,ssbc/patchwork