Adds the base fetch wrapper (client.ts) with JWT auth headers,
automatic token refresh on 401 with request deduplication, and
typed ApiError. Adds auth.ts with login/logout/refresh/listSessions/
terminateSession. Adds authStore (stores/auth.ts) persisted to
localStorage.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
