fix(tui): add missing `theme_list` keybind

[aspiers]

Without this patch, users could not quickly access the theme switcher via key bindings.

This is a problem because theme_list was already documented as working in packages/web/src/content/docs/keybinds.mdx.

So add the theme_list keybind to the Switch theme menu option, allowing binding of quick keyboard access to theme switching as documented.