Thanks for the feedback! I'm still working out the finer details of potential offline currency.
Regarding the game not working when unfocused, this is unfortunately a limitation of the Godot Engine when it comes to web builds. It's a big reason why I stated that the game is intended to be played downloaded. When downloaded, it continues to run in the background just fine.
I understand many people don't want to just blindly download games, though, so I kept the browser version even though it has this limitation.