Clear initial-project hint after first open
Fixes "← Projects" doing nothing. initialProjectIdRef captured the
URL hash at App mount and was never cleared, so every remount of
ProjectList (which happens on every back navigation) consumed it
and immediately re-opened the same project. The user saw the back
click and bounced straight back to the project view.
One useEffect that nulls the ref once activeProject is set
removes the loop without disturbing the "reload restores your
last project" behavior.