Add a rich-text formatting toolbar as the focus-mode header when the
WYSIWYG editor is active: heading dropdown, bold/italic/code, lists, and
link, with a priority+overflow responsive layout and live active-state
highlighting. The toolbar is a self-contained component driven through a
new FormattingController surface routed via EditorContent.
Remove the now-redundant slash-command feature (/todo, /code, /link,
/table) and its "Type / for commands" hint, since the toolbar covers
those actions; the shared suggestion renderer stays for #tag.
