Wow this is very nice.
Just 2 things I thought of though.
1 - Would there be a way to make levels procedurally generated? So that the user can keep playing indefinitely. Having said that your difficulty scaling is quite good for the curated levels, this is probably more for after the users have played all the levels.
2 - Maybe this is worth turning into a PWA and launching on an app/play store to get more exposure. It would have a side effect of looking better on mobile devices.
oh, hey, thanks for playing and for the feedback!
it should be possible to randomly generate levels. generation is easy, the main issue is checking that there aren't trivial solutions anywhere else in the grid. hopefully, if the game gets some more gameplays, i'll consider adding a random challenge mode, as well as some other challenges and maybe additional levels. if this happens, i'll also consider a mobile version.
i've been asked before for pwa of my games, but this would mean using ads for monetization and i'm really not down with ads anymore. hell, google ads banned me ~10 years ago, so...