Function useAnonymousSession

Client hook: fetch and cache anonymous session via SWR. Mirrors the useUser() pattern.

Returns:

• anonymous: AnonymousSession | null (null if no session or fetch error)
• isLoading: boolean (false when error or data loaded)
• error: Error | null (populated on fetch error)
• invalidate: () => void (trigger SWR revalidate)

Uses SWR key: resolved route (option, env var, or default "/auth/anonymous-session") Fetcher: standard fetch, returns null on 204 status, throws on !ok

Test: T7 (hook + provider)