Feature Request: Add ability to disable Assistant

### What

It'd be nice to be able to disable Assistant.

### Why

1. It assumes the end user has an active internet connection.
2. It requires logging into WP.com, which not everyone wants to do.
3. Some people want all data to stay local.

Nothing wrong with keeping it on by default, but something like this should have the option to be disabled.

### How

Add a setting? :^)

Start by locating the Assistant entry points and the existing settings UI or configuration path in the desktop app. Trace how Assistant is enabled today, then define a setting that leaves it enabled by default but allows users to disable it; done means the disabled state prevents Assistant use while preserving the current default.

Written by the indexing model from the issue text.