Monospaced font option.

### What
Monofaced Font option for all notes (in-note - not app-wide)

### Why
A mono face as alternative to the sans would be really good for users who often have date / numeric / tabbed layout so it'd make viewing those much more legible.

Love the app, thanks 👍

No files, tests, or entry points are named. Start by locating how note text fonts are selected and where per-note options are stored, then trace how an option could apply only to the current note; done means users can choose a monospaced font for an individual note without changing other notes or the app-wide font.

Written by the indexing model from the issue text.