Hub: show bottom tab (Home/Search/Login) on the hub landing too
The bottom tab fell under bareChrome → disappeared on the hub-home (no nav
on mobile). Now shown everywhere except auth screens. canCreatePost hardened
against a null site (hub landing without a site context) so the tab does not
crash for logged-in members.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@…>