Hide editor/dashboard cross-link in header when signed out
The contextual nav link (Publications on /editor, Write on /dashboard)
rendered whenever auth was not loading, so signed-out visitors saw a
link into editor-only routes they cannot use. Gate it on signed-in
state instead, matching the identity block and sign-out button.
Adds AppBar render-branch tests covering the link's visibility across
loading / signed-out / signed-in states on both page contexts.