Update FileShare example

[uatuko]

We started on the FileShare example in #67 but it's incomplete and needs updating.

1. Missing features

1.1 Must have

• UX flow to add a new user. Currently this is possible by manually navigating to /sign-up route but should integrate into the listing users UI.
• UX flow to share files with other users.
• Serve static HTML/JS from Go.
• Documentation.

1.2 Should have

• Make the Go server configurable (e.g. listing IP/port, Sentium endpoint).
• Update run make target to generate static HTML/JS before running the Go server.
• Update GitHub Workflows to run tests (and maybe track code coverage?).