Add structured AI courseware APIs

feat(teacher): add reply language selector (zh/en/auto) + per-feature history (last 5); fix 4 issues: language control, history memory, 9 features layout

feat(courseware): AI Teacher Assistant Agent 模块化

Add POST /api/quiz/generate for external sites, docs section 6

IST345 course + multi-TA, AI Quiz API doc, courseDirectory and sidebar updates

Add teacher agent, docs, and project updates; ClareVoiceV1 remains untracked (nested repo)