feat(social): hide social controls on a deleted opponent's seat

A deleted account keeps its seats in every shared game, so its opponents
still saw the add-friend and block controls on the scoreboard (deletion
drops the friendship, so the 🤝 even reappeared for a former friend) and
a chat composer nobody was behind. A friend request sent that way was
accepted by the server and stayed pending forever.

The per-viewer game views now mark such a seat (SeatView.deleted,
resolved beside the seat display names by a batch accounts.deleted_at
lookup) and the client hides every control aimed at it: add-friend,
block, and the chat composer (message + nudge) once no reachable
opponent is left. Live events carry the game domain's seat standings and
so leave the mark unset, so the delta reducers preserve the cached one.
SendFriendRequest and Block against a tombstone are refused with
social.ErrAccountDeleted (410 account_deleted) — the source of truth for
an older client.

docs/FUNCTIONAL.md

@@ -374,7 +374,12 @@ goes **disabled** while a request is pending or was declined and **disappears**
 friends; both controls **disappear and the opponent's name is struck through** once you have
 blocked them. They update in place the moment the relationship changes and stay correct across
 reloads. Applying a block (or friend request) takes effect immediately and is confirmed by a
-live event; a transport failure rolls the control back to its prior state.
+live event; a transport failure rolls the control back to its prior state. Once an opponent
+**deletes their account** there is nobody left to reach, so both controls disappear for that
+seat for good — in the finished game as much as in one still running — and the chat composer
+goes too (message and nudge) when no reachable opponent remains; the chat log and the game
+itself stay readable. Attempting either action anyway (an app version that still shows the
+controls) is refused with *This player has deleted their account*.
 
 Block globally — switch off incoming chat and/or friend requests — or block an **individual
 player**. A per-user block is **one-directional and silent**: you stop receiving everything