refactor: convert the live tab from Redux to React Query (#2006)

Self-wrap the live tab (LiveTab owns useCourseHomeMeta + a new useLiveTabData,
renders TabWithTimer, splits the iframe into a LiveTabContent child). Delete
fetchLiveTab and the now-orphaned shared fetchTab helper (discussion, its other
consumer, converted in the layer below), and remove the Test fetchTab block from
redux.test.js. Part of #1975; closes #2002.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>