Items with isUnlocked = null were evaluated as teacher-locked due to
`?? false` defaulting to false before negation. This caused content to
appear locked immediately after a student marked a lesson complete, even
in free-mode courses.
Changed to `?? true` in both client-side (progression-utils.ts) and
server-side (core/progression.ts) so null/undefined isUnlocked defaults
to unlocked — consistent with the sidebar's own `?? true` guard.
Rotimi Best
c72558f8f1