Allow signed-out visitors to open share links view-only

A share link now opens for someone who is not signed in. They see the document read-only: panels are inert, there is no toolbar, and nothing they do (including paste) changes the document or their own guest canvas. Signing up from that view returns to the same link so it can be redeemed. Signed in, the same link still lets them edit live or duplicate, depending on the mode you set.