Replace the two separate inputs (direct add + discover search) with a
single unified search field. When the user types, results come from
socialStore.searchUsers() across all connected instances. When empty,
the discover grid is shown as before. A user@instance pattern surfaces
a Direct Add action row for sending friend requests to federated users.
Parent FriendsPage no longer owns add-friend state — it all lives inside
AddFriendTab now, keeping the component self-contained.