So sorry to hear about the cat and your tank troubles.
Well there are a number of things which you can do. One is rehome your tank if you do not need it to someone knowledgeable about fish care.
But if you want to keep it then for your snail there are a number if things which you can do to help.
What are your tank parameters?
As a "newb" it would be recommended to start of by doing another water change if you haven't done one this week.
The Goldfish is a big waste producer - it is just what they do.
Spend a little time reading the article in my signature the last line, it will help you with getting started. After reading it and having that stored away for finding answers when you need it, you will have more information than many of the fish store employees in big box stores. YAY!!!
So happy that you are willing to be committed to good care of animals. e
One of the number one issues for the tank is building a big beneficial bacteria colony referred to often as BB.
This means that your filter media is a breeding ground for bb which helps keep your tank healthy and safe for your fish and snail.
Okay, as for the snail...
Get the water checked to he if it is near neutral pH 7, or acidic or alkaline. I am betting towards acidic since there is shell loss in the snail.
Go to the petstore and buy a Cuttlebone - plain no additives, like for birds. rinse in tap water and then just place into the aquarium soft side down towards the water which means hard shell side facing the air. After awhile it will eventually sink.
When it does, just move it towards the rear of the tank to hide it, with the soft side where the snail can glide around and eat off of it. You can lay it horizontally or stick the lower portion in the substrate like a surfboard on the beach.
There is a commonly found food called Hikari Crab Cuisine, This is a food for creatures with exoskeletons.
The other food is Hikari Algae wafers. It is harder to find the small sized wafer package in a small quantity pack but you can find it online. If you plan to have many bottom feeders a larger pack is fine, but these guys are not going to est that much at a time.
They like it pretty well. As they ARE algae eaters they will eat microfilm and tiny things on the glass and plant leaves, in addition to algae on rocks, driftwood and rocks.
Now the water is likely a bit acidic. The number will help us know what to do.
If it is very low it can lead to unstable water parameters.
It would be ideal to know what the tap water pH is and the tank water pH.
Minerals and TDS
build up over time with a low amount of water changes, as water evaporates the minerals and solids stay in the tank.
They snails might like to eat a slice of washed, blanched zucchini and dark greens too as they also contain calcium and minerals good for the snails.