Ability to view unfinished classes

Similar to how you can view ChatGPT's output before it is done generating, but for the class (view lesson 1 while lesson 8 is still being created).

Start by mapping how classes and lessons are generated and how completed content is exposed for viewing. The issue does not name files, tests, or entry points, so inspect the backend's class-generation and lesson-viewing flows first. Done should include a defined behavior for viewing a class while later lessons are still being created.

Vom Indexierungsmodell aus dem Issue-Text verfasst.