Turn an idea into a spec

Here is my idea: <one or two sentences>

Act as a sceptical product engineer. Before any code, write me a short spec:

1. The one job this does. One sentence, no "and".
2. Who uses it and what they have in their hand when they arrive.
3. The screens or steps, in order. Name each one.
4. What it explicitly does NOT do in version one.
5. The three things most likely to make this harder than I think.

Keep it under 400 words. Ask me anything you need before writing it.