option to set standard font type and size

### What

Currently, simplenote shows all raw text with one font type and size. It would be useful to provide an option to render the raw text with other font families (e.g. monospace) and font sizes.

### Why

A user may wish to select a different font family and size to increase the readability of the raw text.

### How

Provide a global option to set the default font type/family and size that is applied to all notes.

The issue names no files, tests, or entry points. Start by locating the raw-text rendering and the global settings or preferences flow, then trace how note display settings are applied. Done means users can choose a default font family and size that apply to all notes.

Written by the indexing model from the issue text.