fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/entranceserver/entrance_server.go

@@ -121,7 +121,7 @@ func (s *Server) handleEntranceServerConnection(conn net.Conn) {
 	if len(pkt) > 5 {
 		data = append(data, makeUsrResp(pkt, s)...)
 	}
-	cc.SendPacket(data)
+	_ = cc.SendPacket(data)
 	// Close because we only need to send the response once.
 	// Any further requests from the client will come from a new connection.
 }