Ability to turn off the toolbar

[rolandwalker]

Description

When the toolbar was made customizable, there was already a default toolbar. To be extra sure we were compatible with any myclirc in the world, the empty value for a toolbar format was made the same as the default toolbar.

This introduces a special value None which makes the toolbar disappear in the UI.

Motivation: the user may prefer the information in, for instance, a tmux window title, or the prompt. Or just a simpler UI.

xref #1684

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Missing regression tests for new toolbar=None behavior
   Change at main.py:1270 adds special handling to disable the toolbar, but the PR only updates config comments (test/myclirc:130) and adds no behavioral test coverage.
   Actionable: add tests that verify:
   • toolbar = None (and case variants like none) disables bottom_toolbar.
   • Empty toolbar (toolbar = '') still keeps default toolbar behavior.
   • Non-None custom toolbar strings still render as before.

No security concerns found in this diff.