The design is really nice, and the note tool works pretty well, I do have a couple comments though.
My first one is that I did a hard sudoku and got a sudoku which has multiple solutions.
My second is that the way the notes are made is great, but one thing which is helpful is when you can have two types of notes: Grid notes (what you already have), and Centered notes (Just writes all the numbers in the middle). This was you can keep track of multiple relationships between cells at a time.
All in all, I must say this is a really clean sudoku experience, and it feels super great to use! Thanks for the fun game!
Oh and also, I love the Solver :)