Nice game! Out of curiosity, are the daily levels built by hand or algorithmically? Is there some way to measure their difficulty computationally, other than just trying to do it yourself or seeing how many people get a perfect score? I'm also working on a grid-based browser game and both those questions have come up for me, I'm keen to see how other people tackle it.
There is a built in map editor, click the hamburger menu.
All the daily levels are built by hand. I struggled to come up with a good random level generator. You can see my feeble attempts in the Edit page (via the hamburger menu) by giving the dice button a few sad clicks.
I did originally try to measure the difficulty computationally by running the solver and timing it, but it didn't really line up with what humans would find difficult. Now I'm just eyeballing it.