[P0.4] Google OAuth sign-in configuration and verification

### Goal
Set up Google Cloud consent screen and Web client, Google-to-Supabase callback URI, Supabase Google provider credentials, app redirect URLs, and verify end-to-end sign-in.

### Dependencies
[P0.1], [P0.2], [P0.3]

### Acceptance Criteria
- Google OAuth client credentials added to Supabase
- Callback URLs properly mapped
- Test sign-in succeeds

### Run
`/setup-auth`

Start with the /setup-auth entry point and review dependencies [P0.1], [P0.2], and [P0.3]. Configure the Google Cloud consent screen and Web client, map the Google-to-Supabase callback and app redirect URLs, add the credentials to Supabase, and verify that a test sign-in succeeds.

Written by the indexing model from the issue text.