WebClient: Single `miden-web-app`, multiple use cases

Currently, each tutorial mandates the user creates a new app (always called `miden-web-app`). This is annoying.

We should either:
1. Amend the instructions to create a different app name, or
2. Use a single app, with setup instructions part of #106. Then, add separate functions to invoke the different tutorial phases:
1. This could be done by adding more UI components (buttons) to trigger the different functions, or
2. Always keep one button, but replace the contents of `app/page.tsx`.

2.i. is a little more complex (e.g. we need to handle cases where tutorial `i-1` was skipped and the user jumped straight to `i`), so I suggest 2.ii should be done.

cc @partylikeits1983 @Dominik1999