List Chat Favorites Chatlist

Favorites tab, assembled server-side in a few batched queries instead of a client-side linear scan over the whole conversation list.

Old path: the client loaded a page of conversations + friends and filtered them by a Set of favorite ids — so favorites outside the loaded page never showed, and the full unified list was rebuilt/sorted on the client. This returns exactly the favorited users, each enriched with their 1:1 last message + unread count, ordered by recent activity. Cost is O(#favorites) with indexed joins (mirrors get_conversations' batched last-message/unread pattern), not O(all conversations).