fix(course): treat null isUnlocked as unlocked in progression logic

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.

apps/dashboard/src/lib/features/course/utils/progression-utils.ts

@@ -43,7 +43,7 @@ function computeProgressionAccess(
   let priorBlockingComplete = true;
 
   for (const item of navigableItems) {
-    const teacherLocked = !(item.isUnlocked ?? false);
+    const teacherLocked = !(item.isUnlocked ?? true);
     const progressionLocked = progressionMode === 'sequential' && !priorBlockingComplete;
 
     let accessible = !teacherLocked && !progressionLocked;

packages/core/src/services/course/progression.ts

@@ -105,7 +105,7 @@ function annotateNavigableAccess(params: {
   for (const item of params.navigableItems) {
     const lessonPolicy = params.lessonPolicyById.get(item.id);
     const exercisePolicy = params.exercisePolicyById.get(item.id);
-    const teacherLocked = !(item.isUnlocked ?? false);
+    const teacherLocked = !(item.isUnlocked ?? true);
     const progressionLocked = params.progressionMode === 'sequential' && !priorBlockingComplete;
 
     let accessible = !teacherLocked && !progressionLocked;