Add a stand-alone mode for the UI

## Why

Using an extension can be a pain in the corporate world with IT blocking a number of custom extensions in the wild.

## What

Add some setting like `mode` or `ui` or something that will launch a stand-alone (Electron?) window when `jam` is invoked with a valid URL. I think the default should still be an extension view. Should be easy enough to implement just render the app in Electron or even use a basic web-server to launch the UI in another window (though I like the elector better I think).

Start by running `jam` with a valid URL and tracing the CLI path to the existing extension view. Compare the proposed Electron window with the alternative basic web-server approach before choosing an implementation. Done means a selectable stand-alone mode opens the UI while the default still uses the extension view.

Written by the indexing model from the issue text.