Function
The lobby leads with the enrolled course watched most recently, then the learner's other courses, then every course they have not joined in catalog order (see courseLobby); each is a CoursePoster. A learner enrolled in nothing also sees a NextUpBar above the heading for the first level course.
The heading renders on the server; the bar and the posters wait for the learner's state. Nothing on the page enrolls: a poster's Enroll opens the course page, where the learner joins.
Available courses: every catalog course as an equal poster, the learner's own courses first.