Invite someone (by Charming handle or email) to one of your apps, OR change the role of someone you already invited. Use this when the user wants to give a specific person access to an app, or to change what an existing collaborator/end-user/viewer can do. Creates a PENDING invitation: the invitee gets no access and sees nothing in their workspace until they accept. Pick a role: `collaborator` (default) can open, edit, and run the app (shared state, same data); `end-user` can open the app and write its data (record entries, save state) but cannot edit the app source; `viewer` can only open and use the read-only surface — viewers cannot edit, run mutating ops, or change anything. If the grantee already has a share, calling this again WITH a `role` changes their role in place (status `updated`, no new invitation, no second email) — this is how you flip a collaborator to an end-user and back; re-sharing WITHOUT a `role` leaves the existing role untouched. No share role can ever share, delete, or transfer the app. The invitee does NOT need a Charming account first — inviting an email with no account stores a pending invitation and emails them a sign-up link; it activates once they register and verify that email (an unknown handle still errors, since there is no address to reach). Revoke anytime with `unshare_app`.