Google OAuth2 authentication for Quarto Hub

[shikokuchuo]

Summary

Adds optional Google Sign-In across the Hub server and and web client.
Auth is off by default and enabled by passing --google-client-id <ID> to the hub server.

Architecture

   Browser (Google Sign-In)                                                  
            │                                                            
       ID token (JWT)                                         
            │
            ▼                                                               
    ┌───────────────────────────────┐
    │       Hub Server (Axum)       │                                       
    │                               │                       
    │  ── auth boundary ──────────  │
    │                               │
    │  REST API    Automerge sync   │
    └───────────────────────────────┘

Auth is enforced at the transport layer (HTTP/WebSocket upgrade) before any request processing begins.
The sync protocol (samod/Automerge) is completely unaware of authentication — unauthenticated connections are rejected before they reach it. This keeps auth concerns out of the collaboration engine entirely.

Server (quarto-hub)

• New auth module that validates Google ID tokens locally using cached public keys (no per-request call to Google).
• All endpoints (/health, /api/*, and WebSocket sync) require a valid token when auth is enabled.
• Operators can restrict access to specific email addresses or domains via --allowed-emails and --allowed-domains.
• Env vars QUARTO_HUB_GOOGLE_CLIENT_ID, QUARTO_HUB_ALLOWED_EMAILS and QUARTO_HUB_ALLOWED_DOMAINS allowed in place of arguments.
• Startup check ensures TLS is accounted for before accepting tokens.

Web client (hub-client)

• When a Google client ID is configured, unauthenticated users see a login screen with Google's standard Sign-In
  button.
• After sign-in, the ID token is stored in the browser and attached to all sync and API connections.
• Token expiry is monitored automatically; silent refresh renews the session without interrupting the user when possible.
• Sign-out button with Google avatar shown in the ProjectSelector header.