feat: write tests for LessonView page (progress tracking, navigation) #633

The LessonView page has no test coverage.\n\nRequired tests:\n- [ ] Lesson content renders correctly\n- [ ] Navigation to next/previous lesson works\n- [ ] Sidebar shows correct lesson list\n- [ ] Current lesson is highlighted in sidebar\n- [ ] Progress is marked complete after finishing lesson\n- [ ] Final lesson shows completion CTA\n- [ ] Milestone submission form renders on milestone lessons