Delete useApiFetch.ts wrapper (returned T|null) and update prep.ts and
prep.test.ts to import useApiFetch from useApi.ts directly, destructuring
{ data, error } to match the established pattern used by all other stores.
Adds usePrepStore (Pinia) for interview prep data: parallel fetch of
research brief, contacts, task status, and full job detail; setInterval-
based polling that stops on completion and re-fetches; clear() cancels
the interval and resets all state. Also adds useApiFetch composable
wrapper (returns T|null directly) used by the store.
