[feature] Save diff style selection

**Version and OS**
v0.50.8 Docker

**Is your feature request related to a problem? Please describe.**

For most of my History diffs I'd like to see the Style "Words" changes and "Ignore Whitespace", but I have to select these every time I load the History.

**Describe the solution you'd like**

An option in the the global Settings to have a default diff style, and perhaps allow overriding in an individual Watch.

The issue names History diffs, global Settings, and individual Watch overrides; start by tracing where the History Style “Words” and “Ignore Whitespace” selections are loaded and stored. Done means a configurable global default is applied when History opens, with an individual Watch override if implemented, and the choices persist across loads.

Written by the indexing model from the issue text.