Respect !no-unauthenticated label on profiles
Profiles that self-apply the !no-unauthenticated label now render a
sign-in prompt for signed-out viewers instead of exposing their posts.
Extends AtprotoClient.getProfile() to surface labels and checks them
before rendering the profile page.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>