That's a really nice front-end! Really good code structure and dev practices too to my JS-nooby eye at least.
A recommendation for the solver part, it seems your using a word trie instead of the more common directed acyclic word graph (DAWG) or the much faster GADDAG. GADDAGs are really cool, +2x faster lookup than DAWGs at 5x space for common Scrabble dictionaries? Even today's CPU cache sizes fit an English GADDAG with ease. [1]
I am working on a project for reinforcement learning for Scrabble, hoping to learn advanced strategies. When I need a front-end, I will definitely reference your project.