feat: add sequential progression and video watch lesson completion

Gate student content by completion order, track per-video watch progress
with seek enforcement, and auto-complete lessons at the configured
threshold. Show ring progress in the lesson header, keep watch progress
on rewatch, and sync progression locks client-side so the next item
unlocks after completion.

apps/dashboard/src/lib/features/course/utils/settings-store.ts

@@ -13,6 +13,7 @@ type CourseSettings = {
   lessonDownload: boolean;
   isPublished: boolean;
   isContentGroupingEnabled: boolean;
+  progressionMode: 'free' | 'sequential';
   callout: TCourseCallout | null;
 };
 
@@ -32,5 +33,6 @@ export const settings = writable<CourseSettings>({
   lessonDownload: false,
   isPublished: false,
   isContentGroupingEnabled: true,
+  progressionMode: 'free',
   callout: null
 });