I think you need to make it more obvious that this is a preview and today you can only use words starting with S.
Changing the initial text in the search to 'Try searching for a word beginning with S...' would go a long way.
I'd also explain the product to people a bit more when on the landing page (without the copy above I would have been baffled) rather than launching them straight into it.
The login / sign up experience is a bit confusing. UX wise the view conflates sign up / login with information about the browser extensions. In my view, it's also a bit early to force people to use passkeys as the only way to login. Regardless, the whole button should be clickable rather than just the key.
The flying text is quite distracting.
On desktop, it's really not obvious that there is a button at the bottom right which has a lot of settings.
I also think there are very few people who would commit to spending $15.64 a month (1564 is Shakespeare's birthdate - nice) without some sort of free trial or at least having more of a go at using the app with more words than just daily one.
Sorry if that's harsh but that's my honest view. As I said, I like the idea overall. There is very little chance I'd spend $15.64 on this as it stands or if ever though.