Add a reset button

[talolard]

At least for the life of V0, we're likely to put the user database in an invalid state and crash everything.

Eventually we'll fix that, but while this is experimental I don't want to be commited to past mistakes. Still we need to give the user an escape hatch when something goes wrong.

So, it needs to be a big red button that deletes the database and recreates it (the structure, not the data).
Also, it needs to be a global exception handler (e.g. a react error boundary) that lets the user press the red button even if everything else dies.