Finish connecting Supabase

The dashboard is running, but it needs a Supabase project for auth and data.

1. Create a free project at supabase.com.

2. Copy .env.example to .env.local and fill in your Supabase URL and keys.

3. Run the migration in supabase/migrations against your project.

4. Set OWNER_EMAILS to your email, then restart the dev server.

See README.md for the full step-by-step.