Post detail: reduce duplicate Sign in CTAs for anonymous users

UX audit finding

Post detail shows Sign in in the global header (UserNav) AND in the comment composer AND on each thread's "Sign in to reply" link. Three identical CTAs on one page feels noisy.

Fix: Keep header auth control; change comment composer to a single softer panel ("Join the discussion — sign in to comment") and hide per-comment reply sign-in when the composer already prompts.