nice! I was looking for a tool like this a while ago. All I could find were either nice paid ones or some pretty crappy free options. I have to shamefully admit I ended up using the free version of Pika and just cropped out the watermark :D Yours looks solid and free. I'd suggest adding a few things like slider for corner radius, shadows, and frames (like macOS, browser, etc.)."
small UI advice: the "Sign in with Google" button has a different border-radius than the rest of the page — might be worth matching it for consistency