Add config.toml option to hide the sidebar

Summary

Add a config.toml option to hide the sidebar.

Rationale

Users can need all terminal width for file panels and the file preview. The current sidebar takes horizontal space even when a user does not need it.

Scope

• Add a Boolean sidebar-visibility option to config.toml (or reuse existing sidebar width options)
• Use the option during layout calculation and rendering.
• When disabled, do not render the sidebar or reserve sidebar width.
• Preserve the current default behavior. The sidebar remains visible unless the user disables it.
• Document the option and its default value.

Related discussion

• #1111
• Requested by: @lazysegtree

Acceptance criteria

• A user can hide the sidebar through config.toml.
• The hidden sidebar consumes no terminal width.
• File panels and file preview use the released width.
• The default configuration keeps the sidebar visible.
• The configuration documentation describes the option.