Add an "in progress" spinner while we do the maths

While a solution is being found, there is no indicator that this is happening. We should add a spinner shown based on the status of the calculation (which already exists in the state in App.js).

Start in App.js, where the calculation status already exists in state, and trace how the solution-finding state controls the displayed UI. Add a spinner that is visible while a solution is being found and hidden otherwise; done means the calculation state clearly communicates that work is in progress.

Written by the indexing model from the issue text.