feat(quickstart): add SSO button using core-3 useSSO hook

[chriscanin]

Summary

Adds an SSO button to the JSOnlyQuickstart sign-in page demonstrating the simplified useSSO() API from core-3.

The button uses the new API where the hook handles everything internally:

const { startSSOFlow } = useSSO()
await startSSOFlow({ strategy: 'oauth_google' })
// Session is now active — no setActive() needed

The SSOButton component is strategy-agnostic and can be reused for any OAuth provider.

Note

Requires the redirect URL to be registered in the Clerk Dashboard. See the useSSO reference for details.

Related

• fix(expo): migrate useSSO to core-3 and fix browser dismissal for OAuth/SSO flows javascript#8140
• docs(expo): update useSSO reference for core-3 and redirect URL requirements clerk-docs#3234

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