Apple sign-in enters rollout

I want people to be able to enter Summ.live with the account they already trust. Apple sign-in is now ready behind a controlled rollout, alongside Google, and the whole login surface feels more official.

Changelog

• Added Apple sign-in support behind a rollout gate.
• Updated sign-in flow to support both Google and Apple login options during rollout.
• Polished the login page and social sign-in button UX for a more official look and feel.