I really love the spirit of a cardgame with deckbuilding being a PnP, but as a cardgame player, I find the deckbuilding UI way too cumbersome to use.
Listing only the card names and their quantities makes it very hard to tell what cards are in a deck, since all of the card's information is now hidden. Ideally, all cards would be visible in their entirety so you can tell how a deck is constructed at a glance. Not being able to see what cards are in your deck makes it hard to know what cards you want to add, and kinda defeats the purpose of a deckbuilder.
The simplified visualization of the cards in the center also seem to have some visual issues, at least on my screen, causing them to be compressed much more than they need to, scrambling the fine lines of the artwork into being unrecognizable. It'd also be nice if you could add cards to a deck with a right-click instead of having to mouse to the bottom-left after selecting a card.
I hope this was constructive, the project seems very professionally put together and I wish you the best going forward.