fix(feedback): show the operator reply only on the player's latest message

A reply was bound to 'the latest message that has a reply', so after the player
read a reply and sent a new (unanswered) message, the old reply kept showing as
'Ответ на ваше последнее сообщение'. Bind it to the single most-recent message
instead: sending any new message immediately drops the previous reply (the new
message has no reply yet), well before the one-week window. Client clears the
reply optimistically on submit; the mock mirrors it; inttest covers the case.

backend/internal/feedback/store.go

@@ -86,25 +86,25 @@ func (s *Store) HasUnreadReply(ctx context.Context, accountID uuid.UUID) (bool,
 // VisibleReply is the operator reply shown back to the player: the reply on their
 // most recent replied message still inside the visibility window.
 type VisibleReply struct {
-	MessageID   uuid.UUID
-	Body        string
-	RepliedAt   time.Time
-	ReplyReadAt sql.NullTime
+	Body      string
+	RepliedAt time.Time
 }
 
-// LatestVisibleReply returns the account's most recent message that carries a reply
-// still visible to the player — not yet delivered, or delivered after cutoff (one
-// week ago). Reports false when there is none.
+// LatestVisibleReply returns the reply on the account's **most recent** message, if
+// that message carries one still visible to the player — not yet delivered, or
+// delivered after cutoff (one week ago). Binding it to the single latest message
+// means that the moment the player sends a newer message the previous reply stops
+// showing (the new message has no reply yet). Reports false when there is none.
 func (s *Store) LatestVisibleReply(ctx context.Context, accountID uuid.UUID, cutoff time.Time) (VisibleReply, bool, error) {
 	var vr VisibleReply
 	var repliedAt sql.NullTime
 	err := s.db.QueryRowContext(ctx,
-		`SELECT message_id, COALESCE(reply_body, ''), replied_at, reply_read_at
-		 FROM backend.feedback_messages
-		 WHERE account_id = $1 AND reply_body IS NOT NULL
-		   AND (reply_read_at IS NULL OR reply_read_at > $2)
-		 ORDER BY created_at DESC LIMIT 1`,
-		accountID, cutoff).Scan(&vr.MessageID, &vr.Body, &repliedAt, &vr.ReplyReadAt)
+		`SELECT COALESCE(reply_body, ''), replied_at FROM (
+			SELECT reply_body, replied_at, reply_read_at FROM backend.feedback_messages
+			WHERE account_id = $1 ORDER BY created_at DESC LIMIT 1
+		) latest
+		WHERE reply_body IS NOT NULL AND (reply_read_at IS NULL OR reply_read_at > $2)`,
+		accountID, cutoff).Scan(&vr.Body, &repliedAt)
 	if errors.Is(err, sql.ErrNoRows) {
 		return VisibleReply{}, false, nil
 	}