apps/web signs in against the server at app.prismark.tech

[hammadmajid]

Part of #3

Question

Make apps/web a client of the server: sign up, sign in, and invite screens against better-auth, served at app.prismark.tech.

• better-auth's React client and the contract client, both configured for the server's URL, with sessions shared on the parent domain.
• Replace the current sign in page (which assumes a code only flow) with sign up, sign in, and the invite landing. Follow DESIGN.md.
• A signed in shell route that proves the session, and nothing more.
• Move the Worker's route from prismark.tech to app.prismark.tech.
• Component tests where they earn their keep; Playwright end to end for the sign up path against the local server.