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/ARCHITECTURE.md

@@ -398,6 +398,16 @@ arrive from a platform rather than completing a mandatory registration).
   full timeline. (A merge that would otherwise leave the survivor with two identities of one
   kind — e.g. each account held a confirmed email — keeps the primary's and journals the
   secondary's with `reason=merge` before dropping it.)
+  A tombstoned account **takes part in no further social exchange**: the per-viewer game
+  views mark its seats (`SeatView.deleted`, resolved by a batch `accounts.deleted_at`
+  lookup beside the seat display names), and the client hides every control aimed at such a
+  seat — add-friend, block, and the chat composer (message + nudge) once no reachable
+  opponent is left. The live events carry the seat standings the game domain holds and so
+  leave the mark unset; the client seeds it from the REST-backed views and the delta
+  reducers preserve it. The server is the source of truth for an older client:
+  `SendFriendRequest` and `Block` against a tombstone return `social.ErrAccountDeleted`
+  (HTTP 410, code `account_deleted`) — otherwise a request would hang pending forever,
+  since deletion already dropped the account's social rows.
   Step-up is a mailed code (`purpose=delete`, no deeplink — a stray click must not delete;
   `ConfirmByToken` refuses a delete token) for an email account, else a typed phrase.
   `last_login_at` / `last_login_ip` are stamped on the cold-load profile fetch (throttled