apps/mobile signs in against any instance

[hammadmajid]

Part of #3

Question

Make apps/mobile a client of any instance: a server address step, then sign in through better-auth's Expo client.

• A first run screen asking for the server address, defaulting to app.prismark.tech, stored securely, changeable from settings.
• better-auth's Expo client and the contract client pointed at that address.
• Replace the current code entry screen with sign in through the methods the server offers, following DESIGN.md and ADR 0009.
• A signed in screen that proves the session, and nothing more.
• Unit tests for the address handling; note what Maestro would cover later.