Layout now renders different nav links based on auth state. Tests were
using a real AuthProvider which resolved to no user, causing them to
look for "My Runs" and "Admin" links that only appear when logged in.
Mock useAuth to test both logged-out and logged-in-as-admin states.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>