Course catalog, quizzes, and ranked stability

What changed

• The course catalog now covers 30 CS courses, with database, operating systems, and data-structure quiz authoring filled out in tracked batches.
• Classroom and quiz media gained reusable crop controls so cover and branding images can be framed before upload.
• Ranked ELO finalization, abandonment handling, leaderboard visibility, and match-found notifications were hardened around server-authoritative writes.