Function join_from_invite_code 

pub async fn join_from_invite_code(
    invite_code_str: &str,
    app_dir: &Path,
    cloudkit_ops: Option<Arc<dyn CloudKitOps>>,
    oauth_cancel: Receiver<bool>,
    clock: ClockRef,
    ids: IdRef,
    make_blob_plan: impl Fn(&LibraryDir) -> Box<dyn BlobPlan>,
    on_status: impl Fn(&str),
) -> Result<Config, JoinError>

Join a shared library using an invite code string.

Handles everything: decode invite, get keypair, build cloud home (including OAuth flows), run the join protocol, and set as active library.