Send Typing Indicator

Show a typing indicator in the conversation (iMessage only).

Works for both 1:1 and group chats: a group id is itself a valid typing target, so every participant sees the indicator. Best-effort: the indicator may be silently dropped if the chat has no recent activity, the recipient isn't on iMessage, or it's RCS-routed (RCS carries no composing state). Auto-expires after a few seconds; no "stop typing" call needed.