Web demo for Private Credentials

[mitschabaude]

add a simple web ui for demoing credentials

ui was mostly written by claude :D

only significant changes to the core library are:

• add the serverNonce to the Presentation type, so that a server can recognize a previously created presentation request
• add a method called Presentation.precompile() which a server can use to compile the spec before creating the actual presentation request (where we already need to know all the claims inputs). This is important because we want to reuse a single spec for many presentations, and the current API assumed it was compiled for each single presentation.
  • I might collapse precompile() and compile() into a single method a future PR