Mode improvements for UI (#3109)

* `--mode=off` now works.

* Add Mode::Off as a persistent CLI option.

* "last" not "auto" as default.

* Commit accidentally unsaved file.

* Whitespace

[ci:skip]

* Mode CLI parse fix

* or offline

* Save mode when it gets changed.

* Fix Offline mode

* Fix up mode operations.

* Make passive default, but not overriding.

* Fix test

* Maybe not everyone wants to run an archive node...