We made 5 or 6 major versions of a similar tool from 2011-now (https://spacedeck.com), in the end it failed to monetize in a significant way. We will probably open-source it soon to provide a continuity path for the (relatively small) core userbase.
Edit: You made some really good UX detail choices: - Grid that snaps on releasing items - Column object (smart!) - Omitting zoom is also probably a good idea (complicates a lot of stuff in the browser: fighting with native zoom, text rendering bugs in firefox etc) - In general being very resistant against feature creep is probably a good idea (in our case, users demanded tons and tons of stuff to be added)