feat(auth): hide Google button when OAuth env is missing

- Export enabledSocialProviders from @ketopath/auth: ['google'] when
  GOOGLE_CLIENT_ID/SECRET are both set, [] otherwise
- sign-in and sign-up pages read enabledSocialProviders server-side and pass
  googleEnabled to their forms; the divider and button disappear when false
- No changes to the Better Auth instance — the Google provider is still
  conditionally registered, this just keeps the UI honest about it

docs/runbooks/google-oauth-setup.md walks through the Google Cloud Console
flow end-to-end (project, consent screen, credentials, redirect URIs, env
injection, verification, troubleshooting, prod considerations).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/auth/src/index.ts

@@ -1,2 +1,2 @@
-export { auth, type Auth } from './server.js';
+export { auth, enabledSocialProviders, type Auth, type SocialProvider } from './server.js';
 export { readAuthEnv, type AuthEnv } from './env.js';