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/channelserver/handlers_clients.go

@@ -77,7 +77,7 @@ func handleMsgMhfListMember(s *Session, p mhfpacket.MHFPacket) {
 			resp.WriteBytes(stringsupport.PaddedString(name, 16, true))
 		}
 	}
-	resp.Seek(0, 0)
+	_, _ = resp.Seek(0, 0)
 	resp.WriteUint32(count)
 	doAckBufSucceed(s, pkt.AckHandle, resp.Data())
 }